Sound Device Problem about RMVB and MP3 Playback

Hello, I have been using Ubuntu for a while. I am still trying to understand it.
I encountered the same problems when I first installed Real Player, problems like, couldn't install the real.deb package automatically or through terminal, real.bin couldn't be executed, package error, etc.. Now I got Real Player working. Then the sound problem occurred, luckily, I used to download a lot of code packages, like gstreamer, a lot of them. I just installed them all. I think one of them was the alsa-oss. I was able to playback rmvb files in Movie Player without video track, just sound. I tried to open rm files in realplayer10 I installed, no sound, nooooo sound. why? That plugin was installed, why it was not working. I went to sound device preference, because I saw something about priority decoder. In preference, I found 2 devices, HDA Nvidia Alsa mixer, Realtek ALC883 OSS mixer. My primary device was HDA Nvidia, so I changed it to Realtck. After reboot, bingo, it was working, I got video and sound in Real Player.
I thought I finished with this problem, but it was not that simple. When I tried to play mp3 in Rethmbox Music Player, no sound, nooo sound. I just realized I had to switch device again in order to playback mp3. In other words, I could not have both rm and mp3 working correctly in the same time.
can you help me?