The player sidebar just loads the SoundCloud website, so it should be any difference compared to SoundCloud in a tab and it's not for me in Opera 112.0.5197.25 64-bit on Windows 11 23H2.
Now, if you have an audio/volume/eq extension installed at the URL opera://extensions, it might not work for the sidebar, which could cause the difference. If that's the case though, the sidebar would be the original, non-enhanced sound.
I guess you could test with "Use graphics acceleration when available" off at the URL opera://settings/system to see if that fixes it.