It would be significantly more complicated, but the proxy technique described by @sixxnet here would likely work - the request for FLAC could be stripped out.
Default All Clients to Max Internet Streaming - #1046 by sixxnet
Jun 22, 2022 15:47:33.294 [0x80b164300] DEBUG - Request: [3.80.144.180:35185 (WAN)] GET /music/:/transcode/universal/decision?hasMDE=1&partIndex=0&mediaIndex=0&directPlay=1&protocol=hls&path=%2Flibrary%2Fmetadata%2F188089 (7 live) TLS Signed-in Token (Username) (Plex for Sonos)
Jun 22, 2022 15:47:33.294 [0x80b164300] DEBUG - [Transcode] TranscodeUniversalRequest: adapting profile with augmentation data: add-transcode-target(type=musicProfile&context=streaming&protocol=hls&container=mpegts&audioCodec=aac)+add-direct-play-profile(type=musicProfile&container=mp4&audioCodec=aac,mp3,alac)+add-direct-play-profile(type=musicProfile&container=mp3&audioCodec=mp3,mp2)+add-direct-play-profile(type=musicProfile&container=flac&audioCodec=flac)+add-direct-play-profile(type=musicProfile&container=ogg&audioCodec=vorbis)+add-direct-play-profile(type=musicProfile&container=asf,wma&audioCodec=wmav2)+add-limitation(scope=musicCodec&scopeName=*&type=upperBound&name=audio.samplingRate&value=48000)+add-limitation(scope=musicCodec&scopeName=*&type=lowerBound&name=audio.samplingRate&value=8000)+add-limitation(scope=musicCodec&scopeName=*&type=match&name=audio.samplingRate&list=8000|11025|16000|22050|24000|32000|44100|48000)+add-limitation(scope=musicCodec&scopeName=*&type=upperBound&name=audio.channels&value=2)+add-limitation(scope=musicCodec&scopeName=*&type=upperBound&name=audio.bitDepth&value=16)+add-limitation(scope=musicCodec&scopeName=*&type=upperBound&name=audio.bitrate&value=1411)+add-limitation(scope=musicCodec&scopeName=wmav2&type=upperBound&name=audio.bitrate&value=355)+add-limitation(scope=musicCodec&scopeName=mp3,aac,vorbis&type=upperBound&name=audio.bitrate&value=320)+add-limitation(scope=musicCodec&scopeName=mp3&type=lowerBound&name=audio.samplingRate&value=16000)
