AVG Free appears to be falsely detecting soundcard drivers, sysaudio.sys, as being infected with the Trojan Dowloader.Delf.BUY
If you delete or move these files to the virus vault, you will disable the sound on your system.
This is the third time in two months Grisoft have done this and they are losing credibility. There are plenty of free and paid for alternatives such as Avast! and AntiVir and it’s difficult to continue recommending AVG.
If you are receiving the message sysaudio.sys is infected with Dowloader.Delf and are concerned then download and run Malwarebytes or follow the Removing a Trojan instructions on the IT Queries webpage to check you aren’t infected with anything nasty.