Make sure audio is stereo. Use audacity to check (even the asset bundled by Scirra some of them are not stereo. if not mistaken).
Before I upgraded to IOS7 my sound doesn't have any issue with IOS6 except the mono-stereo problem which causes safari to hang and etc. Edit the audio to stereo with audacity seems to fix the audio loop in background and some other things.
I just usually need to do this. Kinda like my SOP with my games.
1)Downgrade to 22Khz
2)Upgrade from mono to stereo
3)I usually just import wav(converted in audacity) file and let Construct 2 do the conversion, if construct 2 complain about something, there is something wrong with the audio.
4)Preload the audio.
Make sure you clear safari cache if you did any updates to the game before playing the game.
Firefox 25 and above got some bugs with Construct 2 music files, so just move the audio from music to sound files until this is fixed..