diff --git a/codecs/codecs.js b/codecs/codecs.js
index 00b5735..d972e06 100644
--- a/codecs/codecs.js
+++ b/codecs/codecs.js
@@ -50,6 +50,13 @@ export function getShortMIMEString(info) {
   if (!info) throw `Invalid ProbeInfo`;
   if (!info.streams || info.streams.length === 0) throw `No streams in ProbeInfo`;
 
+  // mp3 files are always considered audio/.
+  if (info.format.format_name === 'mp3') {
+    return 'audio/mpeg';
+  }
+
+  // Otherwise, any file with at least 1 video stream is considered video/.
+  // Otherwise, any file with at least 1 audio stream is considered audio/.
   const type = info.streams.some(s => s.codec_type === 'video') ?
       'video' :
       info.streams.some(s => s.codec_type === 'audio') ? 'audio' : undefined;
@@ -98,16 +105,30 @@ export function getShortMIMEString(info) {
 export function getFullMIMEString(info) {
   /** A string like 'video/mp4' */
   let contentType = `${getShortMIMEString(info)}`;
+  // If MP3, just send back the type.
+  if (contentType === 'audio/mpeg') {
+    return contentType;
+  }
+
   let codecFrags = new Set();
 
   for (const stream of info.streams) {
     if (stream.codec_type === 'audio') {
-      // TODO! At least mp4a!
+      switch (stream.codec_tag_string) {
+        case 'mp4a': codecFrags.add(getMP4ACodecString(stream)); break;
+        // TODO: vorbis.
+        // TODO: opus.
+        default: 
+      }
     }
     else if (stream.codec_type === 'video') {
       switch (stream.codec_tag_string) {
         case 'avc1': codecFrags.add(getAVC1CodecString(stream)); break;
         case 'vp09': codecFrags.add(getVP09CodecString(stream)); break;
+        // We don't handle these as video streams with codecs, so skip them.
+        case 'png':
+        case 'mjpeg':
+          continue;
         default:
           throw `Could not handle codec_tag_string ${stream.codec_tag_string} yet. ` +
               `Please file a bug https://github.com/codedread/bitjs/issues/new`;
@@ -116,7 +137,6 @@ export function getFullMIMEString(info) {
   }
 
   if (codecFrags.length === 0) return contentType;
-
   return contentType + '; codecs="' + Array.from(codecFrags).join(',') + '"';
 }
 
@@ -215,3 +235,22 @@ function getVP09CodecString(stream) {
 
   return frag;
 }
+
+/**
+ * https://developer.mozilla.org/en-US/docs/Web/Media/Formats/codecs_parameter#mp4a
+ * @param {ProbeStream} stream
+ * @returns {string}
+ */
+function getMP4ACodecString(stream) {
+  let frag = 'mp4a.40';
+  switch (stream.profile) {
+    case 'LC':
+      frag += '.2';
+      break;
+    // TODO: more!
+    default: 
+      throw `Cannot handle AAC stream with profile ${stream.profile} yet. ` +
+          `Please file a bug https://github.com/codedread/bitjs/issues/new`;
+  }
+  return frag;
+}
diff --git a/package.json b/package.json
index 886fce5..52e1216 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@codedread/bitjs",
-  "version": "1.0.2",
+  "version": "1.0.3",
   "description": "Binary Tools for JavaScript",
   "homepage": "https://github.com/codedread/bitjs",
   "author": "Jeff Schiller",
diff --git a/tests/codecs.spec.js b/tests/codecs.spec.js
index 42b7cf7..5637e84 100644
--- a/tests/codecs.spec.js
+++ b/tests/codecs.spec.js
@@ -40,6 +40,13 @@ describe('codecs test suite', () => {
       })).to.throw();
     });
 
+    it('detects MP3', () => {
+      expect(getShortMIMEString({
+        format: { format_name: 'mp3' },
+        streams: [ { codec_type: 'audio'}, { codec_type: 'video' } ],
+      })).equals('audio/mpeg');
+    });
+
     it('detects AVI video', () => {
       expect(getShortMIMEString({
         format: { format_name: 'avi' },
@@ -252,4 +259,32 @@ describe('codecs test suite', () => {
       });
     });
   });
+
+  describe('AAC', () => {
+    /** @type {ProbeInfo} */
+    let info;
+
+    beforeEach(() => {
+      info = {
+        format: { format_name: 'mov,mp4,m4a,3gp,3g2,mj2' },
+        streams: [{
+          codec_type: 'audio',
+          codec_tag_string: 'mp4a',
+        }],
+      };
+    });
+
+    it('throws when unknown', () => {
+      expect(() => getFullMIMEString(info)).to.throw();
+    });
+
+    describe('Profile tests', () => {
+      it('recognizes AAC-LC', () => {
+        info.streams[0].profile = 'LC';
+        expect(getFullMIMEString(info))
+            .to.be.a('string')
+            .and.equals('audio/mp4; codecs="mp4a.40.2"');
+      });
+    });
+  });
 });