Nonsense, it is a big deal.
I know it's not so easy, but I would truly love to hear the tones. You are right, it's probably some app that has its own alert tone. Personally, I hate that I can't set them, and have turned most of them off. I'd like a tone for VM, a tone for SMS, and one single, unified tone for all my app alerts. Of particular note, I'd like to change the one for eBay. Hate that one and it always seems to have static in it.
Truly curious at this point as to what's causing this tone. Are you low on battery, maybe?