Myself and many others had a bit of an issue when updating QuickTime to the latest 7.5 release. The updated version played back many of my avi files with video but no audio. The simple solution is to install this QuickTime component, Perian, which is a free open source solution which adds support for various file formats.

Perian enables QuickTime application support for additional Media Types:

• AVI, FLV, and MKV file formats
• MS-MPEG4 v1 & v2, DivX, 3ivX, H.264, FLV1, FSV1, VP6, H263I, VP3, HuffYUV, FFVHuff, MPEG1 & MPEG2 Video, Fraps, Windows Media Audio v1 & v2, Flash ADPCM, Xiph Vorbis (in Matroska), MPEG Layer II Audio
• AVI support for: AAC, AC3 Audio, H.264, MPEG4, and VBR MP3
• Subtitle support for SSA and SRT