WebRTC fix audio downmix to mono in Chrome #2672

[streamthing]

Fixes this issue https://github.com/ossrs/srs/issues/2672

[codecov-commenter]

Codecov Report

Merging #2683 (9857942) into 4.0release (3fc287f) will not change coverage. The diff coverage is n/a.

@@             Coverage Diff             @@
##           4.0release    #2683   +/-   ##
===========================================
  Coverage       60.06%   60.06%           
===========================================
  Files             121      121           
  Lines           50752    50752           
===========================================
  Hits            30486    30486           
  Misses          20266    20266           

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