Well, if you can listen to it then its already being decoded, so you dont actually need to DEcode it.
Your problem here is that mp3's only do stereo, whereas DTS is normally used for 5.1 or 6.1 audio. I think you will have to find a program that turns your surround audio into stereo, THEN you can make mp3's. 
Cant think of any programs, except for the obvious one: Adobe Audition. But thats a very expensive tool for your relatively simple use. Not sure if you can find a cheap app for this. Surround music is not very popular.