On our '06 the turn signal sound came from a speaker that's part of the speedometer. We lost volume on our turn signals, then the volume went away, then later we lost all the buzzers and warning sirens. We had to send the speedometer to Actia to have the speaker replaced, and once that was done, it's as loud as new. Actia turned the speedometer around in a week; very easy to do, and they upgraded our LCD panel to a new series at the same time.
If you decide to go this route, PM me and I'll give you the new contact.
Former Owner, 2006 36MDDS
2010 Arctic Fox 22GK Travel Trailer
2007 Toyota Tundra