I think the hardest part is making the accessory end between Spkr- (pin 9) and Spkr+ (pin 10) because the pin sizes are too small for a standard crimper. Kurt Meltzer at Meltzer Radio Engineering (mre1032@yahoo.com) sells Cable 415 (seller id mre1032) on eBay with the other end unfinished. Be aware that the audio from the accessory port is unbalanced, so likely you'll want (or need) to provide some isolation.
At one time, Kurt sold a Maxtrac audio cable that included an audio transformer. I don't see that he offers one with a transformer for the XPR Mobile, but you could send him a note and ask if you're unsure of skills.