If the video you are trying to watch has sound then you will probably keep getting that error. Even if you could play a video with sound and an mp3 at the same time, (which might be possible but I don't really think so) then the sounds would probably just mix together and you would just hear goobledy gook. Pausing the mp3 while you watch the video might work though.
Thanks, that's all i wanted to hear. Yea never had this kind of problems before in windows, just never experience this before. So if that's the case I guess I can just use one at a time. I'm just pretty much used to multi tasking all the times and do many things at once.