To give you a quick response (as this isn't an area of expertise for me.. to take this for what its worth)
Realize that YouTube (and all similar stream providers) heavily compress and re-encode videos (incl audio) to save on download bandwidth to improve viewer experience (in general). Though, as you've experienced sometimes the result isn't optimal. There is LOTS of online discussion about which video format to use to get yourself the best end result. Granted most folks asking about video, but similar applies to audio. On the video side, I've seen comments about streaming at 1440p vs 1080p for a better result. I have no idea if that impacts audio as well. ... something for you to look into or see if others chime in.. I could be way off base
Regardless, as for troubleshooting, Are you recording or livestreaming?
If you are recording, have you confirmed the OBS recording sounds as desired? If you aren't recording the livestream, and your PC is powerful enough, I'd recommend do so, such that you have a higher quality archive (assuming such matters to you). Regardless, check a local recording (from OBS) to make sure OBS isn't the issue but rather the audio re-encoding YT is doing is causing your problem.
Have you checked YouTube's maximum allowable audio bitrate? I just found
https://support.google.com/youtube/answer/1722171?hl=en
I don't use YT, so can't comment but in general make sure you are using all available Audio bitrate (which per that link I just looked up is a lot higher than I was expecting.) Then, for the discerning ear, installing on the OBS PC the CoreAAC encoder mentioned below _may_ help, if you aren't impressed/ok with the audio quality in the local OBS recording at the higher audio bitrate (beware default/simple OBS settings may be only using something like 128 or 192 kpbs assuming Mono signal?)
OBS Audio related articles
The CoreAudio AAC encoder is purely an audio quality improvement over the built-in OBS AAC encoder. If you'd like to install CA, Here's a guide:
https://obsproject.com/forum/resources/obs-studio-enable-coreaudio-aac-encoder-windows.220/
Again, audio isn't my strong suit. So less that $0.02 worth above. Good luck