Sound on the first beat

Fixes:

• Pressing play could start silent, with sound only arriving after a scrub — and it happened again after every pause and resume. Any timeline change was throwing away decoded audio, so the next play had to re-decode whole clips before making a sound. Audio now starts within a frame of pressing play (about 0.04 seconds, down from two seconds), and clips with fades stream with their volume curve applied live instead of waiting on a full decode.