I've read reviews of Guitar Hero World Tour for the Wii on a number of prominent gaming sites. None criticised the sound quality (with the exception of the player created music) so I've been most frustrated that I've found the sound quality so bad.
I own Guitar Hero III on the Wii and (after the mono sound disc was replaced with the corrected disc) I can't fault the audio. World Tour on the other hand, sounds weak and flat. It's quieter, and when you turn it up, it doesn't get louder, it gets noisier.
To test, I turned World Tour up to a healthy volume, muted my reciever, swapped out Word Tour for GHIII, started a track and unmuted. GHIII was so much louder that I was almost knocked over. GHIII is not just louder, it's punchier and fuller.
Whilst no prominent sources have reported audio problems, many other users have confirmed the poor audio on Wii. Some have suggested that because Wii uses DVD the tracks have to compressed more to fit them on. This argument is flawed by the fact the the Xbox 360 also uses DVD, with more channels of sound, higher polygon count for the models, and higher resolution textures. If World Tour on Wii sound bad because it's on DVD, then with the Xbox 360 version should sound much worse since it has to store more data.
I bought a PS3 a few weeks before buying World Tour for the Wii. I bought the Wii version because it was one less instrument to buy and graphics didn't matter to me. But the sound quality is that bad that I really wish I had bought the PS3 version. The quality is that bad that I have a really hard time enjoying the game.
For reference, Guitar Hero Aerosmith also suffers from weak sound. I don't own it, but my brother does.