audiomixer doesn't negotiate caps correctly for h264 video with mpeg audio

https://www.joshmatthews.net/whiskerstroubled2.mov

The following pipeline plays the audio from this file perfectly: gst-launch-1.0 filesrc location=/Users/jdm/Documents/whiskerstroubled2.mov ! decodebin ! audioconvert ! audioresample ! autoaudiosink

The following pipeline plays no audio at all: gst-launch-1.0 filesrc location=/Users/jdm/Documents/whiskerstroubled2.mov ! decodebin ! audioconvert ! audioresample ! audiomixer ! autoaudiosink

This works fine with a separate video file that's an mp4.