Write out audio metadata frequently. Fixes #169.

If the source was switched among variant playlists that have different audio sampling rates and the initial audio metadata tag skipped, audio would dissapear. Instead, emit an audio meta tag at least once per second so that the decoder can get back on track after a change. Fix typo that caused audio DTS not to be written out properly.

I would normally never propose this sort of PR without an accompanying test case. We're in the middle of a big refactor of the transmux pipeline, however, and I'd rather just write that test for the new infrastructure.