Also, don't know if you're experiencing ticking (common with the 555) but the trick I've seen is to use the CMOS version of the 555, so it doesn't pull as much current on the state changes.
Although, now that I think about it, having the audio path passive means it isn't as closely connected to the same ground as the 555, so that may mean less chance of ticking. You still have to ground the jacks of course, but still, perhaps the passive path is the key to using the 555 without ticking.