Hmm. So Helixplayer's running and looking like it's playing the media? I don't currently have a gentoo install with X so I can only guess from past experience but...
- A long shot, but you've checked the mute/volume control in the player?
- Have you considered that it could be an OSS vs ALSA issue? This can be a real headache (no pun intended).
- Some people report disabling arts in KDE can help/hinder; are you running KDE?
- Depending on your card, window manager etc, you may find that there are quite a few different kinds of volume sliders to play around with in whatever mixer program controlls your system's audio volume
- Is the media you're trying to play embedded in a web-page or playing in the standalone player?
- There are other media players which might be worth trying before you pull your hair out, random example: http://www.videolan.org/vlc/download-gentoo.html
- Have you tried different media sources? (E.g. http://www.bbc.co.uk/radio/)
- Check all your USE flags to make sure everything related to real and audio which you might need is getting compiled. You'd would probably have to have disabled something for things to go wrong that way though.
If you're audio's generally fine and Helixplayer's up on screen you're probably on the home straight.