When I lower osu! overall volume to something low enough (for example, 15% by timing points and 15% by settings) some hitsounds disappear completely.
I can not hear hitnormals, hitwhistles, hitclaps, but hitfinishes and sliderticks are still clearly audible.
If I lower the volume ever more (but not to 0), finishes and ticks disappear too.
It doesn't depend on system volume though; with 100% Windows volume and 10% osu! volume everything is silenced, but with 10% Windows volume and 100% osu! volume I can hear everything clearly.
Also, for example, if I have found out that they are silenced at 15%, and then set 16%, hitsounds come back with a decent volume.
So, I guess that there is some minimum volume which turns off the hitsounds completely.
I can not hear hitnormals, hitwhistles, hitclaps, but hitfinishes and sliderticks are still clearly audible.
If I lower the volume ever more (but not to 0), finishes and ticks disappear too.
It doesn't depend on system volume though; with 100% Windows volume and 10% osu! volume everything is silenced, but with 10% Windows volume and 100% osu! volume I can hear everything clearly.
Also, for example, if I have found out that they are silenced at 15%, and then set 16%, hitsounds come back with a decent volume.
So, I guess that there is some minimum volume which turns off the hitsounds completely.