sounds like you need to start playing around with a wave editor. It's best to do all this manipulation before encoding so you don't lose quality. One thing you could try once you get the hang of the program is extract the left and right channels from each audio file, then take both lefts(original and translated) and create a stereo file from these, i.e. left channel becomes original, right channel becomes translated. Then do the same with both right channels (original and translated). Now you've got 2 stereo files, take them and make them both mono, and then take your 2 mono files and create a new stereo file from them. This will accomplish what you want, i think (having both files playing at the same time). Is that what you're trying to do? Another option is having the original sound in the left channel and the translated in the right. EAC has a decent wave editor built-in, and it's free, it should be enough for your purposes. And there are many others out there, but i wouldn't go overkill with cool edit or something for such simple tasks.