When I've had a rough day or nothing seems to be going right, I put in a CD I've compiled of all recordings on which Charlie sang lead or his harmony vocal is high in the mix. It more or less puts your mind at ease; it does mine. "We--welllllll, now you better run..."
Regarding Poor Man's Gold, as jbgude mentioned earlier, this recording is on Suspicious Minds: The 1969 Anthology, and I can't hear any siren. Elvis sang one line, than laughed a little, and that's the end of the track. I can't remember if I've blasted that with the headphones on before or not, but if there is a siren to be heard on that recording, it is barely audible...at least I hope so.
If you can't hear the siren on that version, then you won't hear it on this version, but I just thought about this and it's kind of neat. That one line is just Elvis singing a cappella...so it doesn't seem that a siren disrupted an actual take. Anyway, besides the one line from Elvis, all RCA ever recorded was the backing track. Merijn van der Meer put the brief vocal to the appropriate music, which was a good idea...one that should have occurred to BMG. (A possibility is that Elvis actually was singing to the pre-recorded track, but BMG simply chose to present it in a cappella fashion. I don't think that's the case, but I'm no sessions wiz. A check of the sessions chronology would probably answer that real quick, but I'm lazy.) You can hear that short goodie here: http://www.evdmeer.dds.nl/homepage.html