diff --git a/src/main/java/gov/nasa/pds/tools/util/EncodingMimeMapping.java b/src/main/java/gov/nasa/pds/tools/util/EncodingMimeMapping.java
index 7af4f609e..f8f784046 100644
--- a/src/main/java/gov/nasa/pds/tools/util/EncodingMimeMapping.java
+++ b/src/main/java/gov/nasa/pds/tools/util/EncodingMimeMapping.java
@@ -9,6 +9,7 @@ public enum EncodingMimeMapping {
   GIF(Arrays.asList("gif")),
   J2C(Arrays.asList("j2c", "jp2", "mj2", "mjp2")),
   JPEG(Arrays.asList("jpg", "jpeg")),
+  M4A(Arrays.asList("m4a")),
   MP4(Arrays.asList("mp4")),
   NEF(Arrays.asList("nef")),
   PDF(Arrays.asList("pdf")),
@@ -34,6 +35,7 @@ public static EncodingMimeMapping find (String encoding) throws IllegalArgumentE
       if (encoding.equalsIgnoreCase("JP2")) return J2C;
       if (encoding.equalsIgnoreCase("JPEG")) return JPEG;
       if (encoding.equalsIgnoreCase("MMM ODR Stream")) return DAT;
+      if (encoding.equalsIgnoreCase("M4A/AAC")) return M4A;
       if (encoding.equalsIgnoreCase("MP4/H.264")) return MP4;
       if (encoding.equalsIgnoreCase("MP4/H.264/AAC")) return MP4;
       if (encoding.equalsIgnoreCase("MSSS Camera Mini Header")) return DAT;
diff --git a/src/main/java/gov/nasa/pds/tools/validate/ProblemType.java b/src/main/java/gov/nasa/pds/tools/validate/ProblemType.java
index 4155f961d..0ce1c196e 100644
--- a/src/main/java/gov/nasa/pds/tools/validate/ProblemType.java
+++ b/src/main/java/gov/nasa/pds/tools/validate/ProblemType.java
@@ -252,6 +252,7 @@ public enum ProblemType {
 
   NON_MP4_FILE("warning.file.not_mp4_mimetype"),
   NON_WAV_FILE("warning.file.not_wav_mimetype"),
+  CONTENT_VALIDATION_UNSUPPORTED("warning.validation.content_validation_not_yet_supported"),
 
   NON_REGISTERED_PRODUCT("warning.product_not_registered"),
 
diff --git a/src/main/java/gov/nasa/pds/tools/validate/rule/pds4/FileReferenceValidationRule.java b/src/main/java/gov/nasa/pds/tools/validate/rule/pds4/FileReferenceValidationRule.java
index 1660e0e33..a9481a21b 100644
--- a/src/main/java/gov/nasa/pds/tools/validate/rule/pds4/FileReferenceValidationRule.java
+++ b/src/main/java/gov/nasa/pds/tools/validate/rule/pds4/FileReferenceValidationRule.java
@@ -493,7 +493,15 @@ private boolean validateFileAreaDefinitionAndContent(String fileName, TinyNodeIm
           } else if (doctype.equalsIgnoreCase("MP4/H.264")){
             new AudioVideo(this.getListener(), target, urlRef).checkMetadata (false, true);
           } else if (doctype.equalsIgnoreCase("M4A/AAC")){
-            new AudioVideo(this.getListener(), target, urlRef).checkMetadata (true, false);
+            // Content validation for M4A/AAC is not yet fully implemented.
+            // Consult PDS4 experts to determine how to validate these products.
+            this.getListener().addProblem(new ValidationProblem(
+                new ProblemDefinition(ExceptionType.WARNING,
+                    ProblemType.CONTENT_VALIDATION_UNSUPPORTED,
+                    "Content validation for encoding type 'M4A/AAC' is not yet supported by this version of validate. "
+                        + "Please consult PDS4 experts for guidance on validating M4A/AAC audio products: "
+                        + urlRef.toString()),
+                target));
           } else if (doctype.equalsIgnoreCase("WAV")) {
             new AudioVideo(this.getListener(), target, urlRef).checkWavHeader();
           } else if (!doctype.equalsIgnoreCase("UTF-8 Text")
diff --git a/src/test/resources/features/4.2.x.feature b/src/test/resources/features/4.2.x.feature
index a2303f1e8..a8206af31 100644
--- a/src/test/resources/features/4.2.x.feature
+++ b/src/test/resources/features/4.2.x.feature
@@ -12,4 +12,7 @@ Feature: 4.2.x
 | 1601 | 1 | "github1601" | "-R pds4.bundle --skip-context-validation -t {datasrc}/bundle_test_1601.xml" | "summary:totalErrors=1,summary:referentialIntegrity:failed=1,summary:messageTypes:error.integrity.member_not_found=1" |
 | 1601 | 2 | "github1601" | "-R pds4.bundle --skip-context-validation --skip-strict-collection-membership -t {datasrc}/bundle_test_1601.xml" | "summary:totalWarnings=1,summary:messageTypes:warning.integrity.member_not_found=1" |
 
+# github1635: M4A/AAC should be a recognized encoding type; content validation not yet supported → WARNING not ERROR
+| 1635 | 1 | "github1635" | "--skip-context-validation -t {datasrc}/audio_m4a.xml" | "summary:totalWarnings=1,summary:messageTypes:warning.validation.content_validation_not_yet_supported=1" |
+
 #end
diff --git a/src/test/resources/github1635/audio_m4a.xml b/src/test/resources/github1635/audio_m4a.xml
new file mode 100644
index 000000000..c95fc6244
--- /dev/null
+++ b/src/test/resources/github1635/audio_m4a.xml
@@ -0,0 +1,62 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<?xml-model href="https://pds.nasa.gov/pds4/pds/v1/PDS4_PDS_1K00.sch" schematypens="http://purl.oclc.org/dsdl/schematron"?>
+
+<Product_Observational xmlns="http://pds.nasa.gov/pds4/pds/v1"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="
+    http://pds.nasa.gov/pds4/pds/v1 https://pds.nasa.gov/pds4/pds/v1/PDS4_PDS_1K00.xsd">
+
+    <Identification_Area>
+        <logical_identifier>urn:nasa:pds:audio_example:data:sample_m4a_audio</logical_identifier>
+        <version_id>1.0</version_id>
+        <title>Sample M4A/AAC Audio File</title>
+        <information_model_version>1.20.0.0</information_model_version>
+        <product_class>Product_Observational</product_class>
+    </Identification_Area>
+
+    <Observation_Area>
+        <Time_Coordinates>
+            <start_date_time>2024-01-01Z</start_date_time>
+            <stop_date_time>2024-01-01Z</stop_date_time>
+        </Time_Coordinates>
+        <Investigation_Area>
+            <name>Test Mission</name>
+            <type>Mission</type>
+            <Internal_Reference>
+                <lid_reference>urn:nasa:pds:context:investigation:mission.mars2020</lid_reference>
+                <reference_type>data_to_investigation</reference_type>
+            </Internal_Reference>
+        </Investigation_Area>
+        <Observing_System>
+            <Observing_System_Component>
+                <name>Test Instrument</name>
+                <type>Instrument</type>
+                <Internal_Reference>
+                    <lid_reference>urn:nasa:pds:context:instrument:mars2020.camera</lid_reference>
+                    <reference_type>is_instrument</reference_type>
+                </Internal_Reference>
+            </Observing_System_Component>
+        </Observing_System>
+        <Target_Identification>
+            <name>Mars</name>
+            <type>Planet</type>
+            <Internal_Reference>
+                <lid_reference>urn:nasa:pds:context:target:planet.mars</lid_reference>
+                <reference_type>data_to_target</reference_type>
+            </Internal_Reference>
+        </Target_Identification>
+    </Observation_Area>
+
+    <File_Area_Observational>
+        <File>
+            <file_name>sample.m4a</file_name>
+            <creation_date_time>2024-01-01T00:00:00</creation_date_time>
+        </File>
+        <Encoded_Audio>
+            <local_identifier>sample_audio</local_identifier>
+            <offset unit="byte">0</offset>
+            <encoding_standard_id>M4A/AAC</encoding_standard_id>
+            <description>Sample M4A audio file</description>
+        </Encoded_Audio>
+    </File_Area_Observational>
+</Product_Observational>
diff --git a/src/test/resources/github1635/sample.m4a b/src/test/resources/github1635/sample.m4a
new file mode 100644
index 000000000..16295020f
Binary files /dev/null and b/src/test/resources/github1635/sample.m4a differ