Yea, you really want a NON-polarized cap. A Non-polarized one will hold up longer as it can tolerate being subjected to what is essentially a reverse polarity condition when audio is passed thru it.

Should you use this cap, failure will take one of two forms:

the cap dead shorts - which is like not having it in there at all - and the system will sound like it does now.
The cap opens up - meaning that speaker goes dead.

Neither failure is likely to cause the cap to burst - so you might try it for now and see how you like the sound.

If you go up several posts, you'll find the MOUSER number for the caps I ordered (330's and 470's) - you can order from them online or with a phone call. I'v bought from them many times since 2000 and have not had a bad part OR a big delay in shipping yet! A search of their inventory will show values of 100 uF to 1,000 uF - they have most values in stock.