Hi GuidoSome things to check to try and eliminate the problem.
1) If you export to video without using .popVideos or other videos in your scene do you get audio?
2) Have you tried exporting to WMV instead of AVI? This will eliminate a possible codec issue.
3) Try using another program to output to video, for example Windows Movie Maker. Do you get sound in the exported video using other software.
4) In addition, check to see if you have updated sound drivers for your computer? Installing these may resolve the issue. You should be able to find them on your computer manufacturers website.
Please give this a try and see if it helps solve the problem.