If you're running a bleeding-edge system (Arch, Gentoo, Fedora...) and recently came into issues, such as:
1. The game running twice as fast as it should be
2. Really crackling sound
Run osu! from a terminal and watch output.
If you see a lot of:
ALSA lib pcm.c:7843:(snd_pcm_recover) underrun occurred
You MUST downgrade pulseaudio.
For Arch users
bahamete@pc ~ $> cd /var/cache/pacman/pkg/
bahamete@pc /var/cache/pacman/pkg $> ls | grep pulseaudio
pulseaudio-4.0-2-x86_64.pkg.tar.xz
pulseaudio-4.0-5-x86_64.pkg.tar.xz
pulseaudio-4.0-6-x86_64.pkg.tar.xz
pulseaudio-5.0-1-x86_64.pkg.tar.xz
pulseaudio-alsa-2-2-any.pkg.tar.xz
bahamete@pc /var/cache/pacman/pkg $> sudo pacman -U pulseaudio-4.0-6-x86_64.pkg.tar.xz
You must do the same with libpulse and lib32-libpulse. It seems recent Pulseaudio update broke 32-bit ALSA compatibility, so do not update Pulse until we get pulseaudio-alsa update or something. I'll report back when this happens.