Three coupled changes after still-choppy reports on UA-gated NACK build:
1. 510k → 320k Opus everywhere in music mode. 320k is the transparent
listening-test threshold (Audible masters at this). 510k is spec
ceiling but real-world uploads can't sustain it alongside concurrent
screen 6M + camera 4M without dropping audio packets. Choppy at 510k
persisted after NACK rollback, so bitrate is the next dial.
2. Drop a=maxptime:120 advertisement. Tried to give Opus larger encode
windows for cleaner music at same bitrate. Chrome publishers may have
actually honored it and packed at higher ptime, which the receiver
side handled poorly (jitter buffer + playoutDelayHint=0.7 weren't
tuned for >20ms packetization). Default 20ms stays.
3. Fix preferStereoOpus regex no-op on Firefox publishers. Previous
matcher anchored on minptime=10 which Chrome emits but Firefox does
not — so for years, Firefox-published mic SDPs went out unmunged:
no stereo=1, no maxaveragebitrate cap, no fullband pin, no NACK gate.
Now: scan rtpmap for all opus/48000/2 PTs, update existing fmtp lines
in place, or insert a fresh fmtp if none exists (Firefox's case).
This is why fox saw "audio NACK as sender" diagnostic never fire on
Firefox publish — the function early-returned before reaching it.
Kept: fullband pins (no bandwidth cost), UA-gated NACK code (honest
per-browser advertisement), speaker output picker, 700ms playoutDelayHint.
Russell Ballestrini
aed8484293