It's a complicated project. You should take it to some parts. It's my suggestion and would be happy if other experts improve it.
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Background music in games is typically mp3/ogg/aac/etc track, sound effects are mostly use wav format or similar. As for me it seems that some codecs are missed, for example music could use wma or some other format that is not installed by default. You can try to install FFmpeg, or VLC which contain many audio/video codecs.
Try playing the downloaded file with Firefox using File->Open.
If doesn't work, check that you have the AAC codecs installed using ffmpeg -codecs.
If playing the local file does work, then, as outlined here you should start Firefox with MOZ_DISABLE_CONTENT_SANDBOX=1 env variable, or whitelist the directory where the codec libraries are located in, e.g. ...