Well strangely, mine has sorted itself out. No idea how.
I got home the other day a tried my wife's 4s, that had no problems, so to me that cancelled out an iOS6 issue as we are both running the same software.
Over the last few days I have been transferring my contacts from Goolge Exchange to iCloud and getting that running nicely, so in the process I setup 'My Info' and added it to Siri.
I have been using Siri quite a bit in trying to get it to pronounce certain contacts names correctly, all this mind you without using the earbuds but just directly to the phone.
I was with Frank in not wanting to 'Reset All Settings' straight away, that would be a headache.
Today though, I plugged in the earbuds, listened to a little music, asked Siri to perform a couple of tasks and then immediately checked the volume of my Alerts, they were fine, full volume.
I moved the slider to about have inside the Music app and checked my Alerts volume again, once again they were perfectly correct, full volume.
So, again, no idea how this happened but my 5 is now tickety boo.
Regards..,