You can't access the PM function until you have 20 posts it is one of the anti-spammer measures.
Only .txt, .log and image files can be attached (.gif/.jpg/.png) trying to change the file type to circumvent that measure is likely to corrupt the file anyway.
As for the sound, if you have deleted the file, a repair of avast should/may bring it back:
Win7/Vista - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
You may need to reboot after the repair.
EDIT: Typo.