This issue just started occurring though to my knowledge nothing about my computer has changed. I record gameplay with fraps and then compress it using virtual dub. The codec I use is Xvid 720p and then I bring my gameplay into Adobe Premier to add commentary and edit it. This has always worked but just today suddenly my imported videos into Adobe Premier don't import audio as well, only video. When I play the files in VLC they have audio, but Adobe Premier does not import an audio track at all. This only afflicts my Xvid compressed videos too, my raw recordings import both the video and audio.
In the cache preferences (Premiere Pro > Preferences > Media) you’ll see a Clean button. Unfortunately, this function works counter to what might be expected. The Clean button deletes cache files associated with files that are currently not online. It ignores cache files related to your currently open project." Source: http/mediastorm.com/blog/2013/04/01/mediastorm-guide-to-the-adobe-premiere-pro-media-cache-database/
Close Premiere after you clean the cache and then restart Premiere and try importing the .avi files again.