Sorry not possible. A SVCD disk has a specific format and is made for Video only. You cant even add another movie to a pre-burnt SVCD either.
However, you could "encode" your WAV files to an SVCD Mpeg without any video for each audio track. Then use VCDEasy to load in the first main movie, followed by each mpg audio track and burn out. You could use the Next Chapter button of your remote to jump to the next track, or go back etc. But they will be in MPG format, ir MP2