diff --git a/demux/codec_tags.c b/demux/codec_tags.c
index e520161b87..c74f66f30c 100644
--- a/demux/codec_tags.c
+++ b/demux/codec_tags.c
@@ -18,6 +18,8 @@
 #include <libavformat/avformat.h>
 #include <libavcodec/avcodec.h>
 #include <libavutil/common.h>
+#include <libavutil/intreadwrite.h>
+
 #include "codec_tags.h"
 #include "stheader.h"
 #include "common/av_common.h"
@@ -48,14 +50,50 @@ static const char *lookup_tag(int type, uint32_t tag)
     return id == AV_CODEC_ID_NONE ? NULL : mp_codec_from_av_codec_id(id);
 }
 
+static const unsigned char guid_pcm[16] =
+    {0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00,
+     0x80, 0x00, 0x00, 0xaa, 0x00, 0x38, 0x9b, 0x71};
+static const unsigned char guid_float[16] =
+    {0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00,
+     0x80, 0x00, 0x00, 0xaa, 0x00, 0x38, 0x9b, 0x71};
+// Corresponds to FF_MEDIASUBTYPE_BASE_GUID (plus 4 bytes of padding).
+static const unsigned char guid_ffext[16] =
+    {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00,
+     0x80, 0x00, 0x00, 0xAA, 0x00, 0x38, 0x9B, 0x71};
+
 static void map_audio_pcm_tag(struct mp_codec_params *c)
 {
+    // MS PCM, Extended
+    if (c->codec_tag == 0xfffe && c->extradata_size >= 22) {
+        // WAVEFORMATEXTENSIBLE.dwChannelMask
+        uint64_t chmask = AV_RL32(c->extradata + 2);
+        struct mp_chmap chmap;
+        mp_chmap_from_waveext(&chmap, chmask);
+        if (c->channels.num == chmap.num)
+            c->channels = chmap;
+
+        // WAVEFORMATEXTENSIBLE.SubFormat
+        unsigned char *subformat = c->extradata + 6;
+        if (memcmp(subformat + 4, guid_ffext + 4, 12) == 0) {
+            // libavformat extension.
+            c->codec_tag = AV_RL32(subformat);
+            c->codec = lookup_tag(c->type, c->codec_tag);
+        }
+        if (memcmp(subformat, guid_pcm, 16) == 0)
+            c->codec_tag = 0x0;
+        if (memcmp(subformat, guid_float, 16) == 0)
+            c->codec_tag = 0x3;
+
+        // Compressed formats might use this.
+        c->extradata += 22;
+        c->extradata_size += 22;
+    }
+
     int bits = c->bits_per_coded_sample;
     int bytes = (bits + 7) / 8;
     switch (c->codec_tag) {
     case 0x0:       // Microsoft PCM
     case 0x1:
-    case 0xfffe:    // MS PCM, Extended
         if (bytes >= 1 && bytes <= 4)
             mp_set_pcm_codec(c, bytes > 1, false, bytes * 8, false);
         break;
diff --git a/demux/demux_mkv.c b/demux/demux_mkv.c
index 9515cd10c5..97c7d3e09b 100644
--- a/demux/demux_mkv.c
+++ b/demux/demux_mkv.c
@@ -1549,12 +1549,12 @@ static int demux_mkv_open_audio(demuxer_t *demuxer, mkv_track_t *track)
 
     unsigned char *extradata = track->private_data;
     unsigned int extradata_len = track->private_size;
-    uint64_t chmask = 0;
 
     if (!track->a_osfreq)
         track->a_osfreq = track->a_sfreq;
     sh_a->bits_per_coded_sample = track->a_bps ? track->a_bps : 16;
     sh_a->samplerate = (uint32_t) track->a_osfreq;
+    mp_chmap_set_unknown(&sh_a->channels, track->a_channels);
 
     for (int i = 0; mkv_audio_tags[i][0]; i++) {
         if (!strcmp(mkv_audio_tags[i][0], track->codec_id)) {
@@ -1581,10 +1581,11 @@ static int demux_mkv_open_audio(demuxer_t *demuxer, mkv_track_t *track)
         extradata = track->private_data + 18;
         extradata_len = track->private_size - 18;
         sh_a->bits_per_coded_sample = track->a_bps;
+        sh_a->extradata = extradata;
+        sh_a->extradata_size = extradata_len;
         mp_set_codec_from_tag(sh_a);
-        // WAVEFORMATEXTENSIBLE.dwChannelMask
-        if (sh_a->codec_tag == 0xfffe && extradata_len >= 6)
-            chmask = AV_RL32(extradata + 2);
+        extradata = sh_a->extradata;
+        extradata_len = sh_a->extradata_size;
     } else if (!strcmp(track->codec_id, "A_PCM/INT/LIT")) {
         bool sign = sh_a->bits_per_coded_sample > 8;
         mp_set_pcm_codec(sh_a, sign, false, sh_a->bits_per_coded_sample, false);
@@ -1704,10 +1705,6 @@ static int demux_mkv_open_audio(demuxer_t *demuxer, mkv_track_t *track)
     if (!sh_a->codec)
         goto error;
 
-    mp_chmap_from_waveext(&sh_a->channels, chmask);
-    if (sh_a->channels.num != track->a_channels)
-        mp_chmap_set_unknown(&sh_a->channels, track->a_channels);
-
     const char *codec = sh_a->codec;
     if (!strcmp(codec, "mp2") || !strcmp(codec, "mp3") ||
         !strcmp(codec, "truehd"))