Fix audio and playback issues

[rbouqueau]

Still some conformance issues. We are limited by our toolset.

Should implement the first step of #13 and fix #14.

[jpiesing]

. We are limited by our toolset.

Please could you give more details about these limits? What is blocked?

[rbouqueau]

Please could you give more details about these limits? What is blocked?

Points that I investigated but could not solve:

• both the @timeline with MaxSegmentDuration and the sum of segment duration doesn't match the period duration (FFmpeg)
• check during side experiments report inconsistent durations between segments and manifest (timeline) (FFmpeg)
• the segments and manifest expressiveness are limited (that's why the script manipulates the manifest (while not checking the version of the tool that generates the manifest -> strong weakness and maintenance burden)) (FFmpeg + having tests)
• lmsg/meta box warnings (FFmpeg + investigate conformance.dashif.org)
• mixing avc1/avc3 silently (FFmpeg)
• express role on audio (FFmpeg)
• on the conformance side: when investigating I could generate streams with fairly different structures that were compliant - which lowered my confidence in the checker. We should do manual tests along the way and report at https://github.com/Dash-Industry-Forum/DASH-IF-Conformance/issues. Note that having an API for conformance.dashif.org would help for this work.
• audio: support for absence of PCE (and maybe other) not supported (FFmpeg + investigate the audio list of requirements for w.r.t. this repo)
• encryption: not support by FFmpeg

I have ideas to solve most of the points but imho we need consolidate the result from the this PR first.