Use only the recommended voltage batteries. 7.4 v is passable, but no more.
More voltage will not give higher speeds and louder sound, and might damage the elecronics, which are expensive to replace.Normally, especially on the Tigers, you would want to slow them down a little using a gear down on the transmissions.
You can use a different speaker, as long as the impedence is 4 ohm. The stock speaker is 5w.
Sev