I was recently having trouble with sound and RealPlayer10 on Linux. It turns out that RealPlayer10 (and Helix Player) only support the older OSS sound drivers (rather than the newer ALSA drivers). I was on 2.6.6. I downloaded and this time only compiled the OSS drivers and not the ALSA ones. Be aware that you should not (perhaps cannot) load both OSS and ALSA drivers.