Sounds like both speakers are blown and must be replaced...
I just read the original post from Mailman more carefully and I can only speculate that when the two yellow wires were touched together, which I assume carried the line-out speaker signal from the amp, it may have fried the amp, hence why the audible "pop" was the last thing he heard. The only way to know for sure is to connect the speakers to another audio source and see if they work at all. Given the age of the sling, I wouldn't rule out a cracked magnet, at least in the right speaker, hence why it sounded so bad before it died. If I'm correct about the fried amp, there's a good chance the left speaker still works, but if you replace one of them, you might as well replace them both and you'll likely need a new amp, too. By the same token, if you connect a multimeter to those yellow wires, you should see a DC signal of about 4 to 8 volts, if the amp is still working. If you don't see any volts at all, then the amp has given up the ghost. Hope all this helps.