Solution !
I had the same problem. All my AVI conversions had No Sound.
I had the latest version of MPEG Streamclip (1.9.3b1) - "Didn't work"
I had the latest version of Perian - "Still didn't work"
The links above to version 1.9.1, and on a Google Search, don't provide the old application
Then, after downloading 1.9.2 I tried again - "Still didn't work"
...but then I noticed that when I Converted to Quicktime (creates .mov file), the sound setting always said "Uncompressed"
I simply went back to Convert to AVI, and selected the Sound setting, changing it to "Uncompressed"...
... it works! 🙂
... let me know how you get on !