While opening VirtualDub I get the error :

VirtulaDub has detected an improper VBR audio encoding in the source AVI file and will rewrite the audio header with standard CBR values during processing for better compatibility. This may introduce up to 0 ms of skew from the video stream. If this is unacceptable, decompress the entire audio stream to an uncompressed WAV file and recompress with a constant bitrate ancoder.(bitrate: 128.0 kbps)

what do I have to do in the virtualdub to fix it ? (I cant convert it in the TMPGEnc)
How do I decompress/compres the wav file?