Browsers such as Safari and IE11 support playing files containing multiple audio tracks, and allow the user to select which channel they wish to listen to. This is needed for files which have tracks in different languages, or tracks for different speaker configurations such as multi-channel surround sound vs mono.
This is experimental in Firefox as of version 33 (and in Chrome as of version 45). Support depends on two things: support for the AudioTrackList and VideoTrackList W3C spec, and support for actual multi-track media playback.
In Firefox, support for the spec can be tested by enabling the media.track.enabled setting in about:config. Support for handling multiple tracks tracked separately here https://bugzilla.mozilla.org/show_bug.cgi?id=1057233
This can be tested in Chrome by enabling the enable-experimental-web-platform-features flag https://bugs.chromium.org/p/chromium/issues/detail?id=851160
Track stable support across browsers here: https://caniuse.com/audiotracks