The on-access scanners, Web Shield, Internet Mail, Network Shield, Instant Messaging and P2P shields are all designed to keep malware out, to stop it getting established, with the Standard Shield as a last line of defence.
Now not all files are scanned by the standard shield, e.g. those not commonly infected, when you bump up the sensitivity from Normal to High then all get scanned but that could have an impact on performance depending on your system specs.
So for me a manual weekly on-demand scan, Standard sensitivity and without Archives is enough and doesn't take that long. This does however, depend on your system spec, the volume of data on your HDD that would need to be scanned.
Generally audio files present a limited risk and that is more say an mp3 file being modified to exploit media player vulnerabilities, so it would be possible to exclude certain media files from being scanned by the on-demand scans (but not the on-access scanner).
So it entirely depends on your system, for me I still do a weekly on-demand scan.