Some more tests which could use a speedup: AudioOutputDelegateTest.Error (1000 ms), MojoRendererTest.GetMediaTime (1500 ms), PipelineIntegrationTest.PlaybackWithAudioTrackDisabledThenEnabled (2900 ms, but maybe it just takes that long, it's an integration test after all), AudioRendererMixerBehavioralTest.MixerPausesStream/0 (1200 ms). There are also tests that use fake streams (like AudioOutputResamplerTest.* and many others) which could be reduced by using mock time.

For reference, I wrote https://chromium-review.googlesource.com/c/chromium/src/+/962446 earlier.

This could be good work for someone to learn about tasks, run loops, et c.