I have never been able to consistently replicate this nor manage to record it, but this has been happening for a while so here it goes...
If you're in the song selection screen on borderless, sometimes when unfocusing osu! (going to a different window) and then re-focusing osu! again the song suddenly plays ALOT louder than it's supposed to for a second, and then goes back to its normal volume. If you switch to a different song it also suddenly plays alot louder for a second. It stops doing this after ~10 seconds, and I can't consistently replicate this either. I'm using 40% master , 75% music and 100% effect volume.
If you're in the song selection screen on borderless, sometimes when unfocusing osu! (going to a different window) and then re-focusing osu! again the song suddenly plays ALOT louder than it's supposed to for a second, and then goes back to its normal volume. If you switch to a different song it also suddenly plays alot louder for a second. It stops doing this after ~10 seconds, and I can't consistently replicate this either. I'm using 40% master , 75% music and 100% effect volume.