You can keep the beginning of the track safe without having to crop it out by doing it this way in Audacity.
1. Highlight the starting point and go all the way to the end of the track that you want the music to stop at and trim it.
2. Then go to your looping point, and look at the Audio Position. It should say something like **.*** s.
3. Have that in your calculator and times it by the Hz (EX. 34.035 * 44100 = 1500943 (ignore the number past the decimal))
4. Once that is done, go into File --> Edit Metadata and look for a blank column and put in LOOPPOINT and in the value column, the number you calculated.
5. Press OK and export it as an OGG File and use SLADE to make a wad and insert the music into the entry list and set a lumpname for it. To find out which lumpname plays on what tune number, use this link as a guide.
wiki.srb2.org/wiki/MusicSlots
If everything worked out OK, the music should loop in SRB2 when it reaches the end.