Try exporting the audio only using QuickTime from the existing movie file from iMovie.
Export as wav/aiff
If audio levels sound fine use Audacity which is free on the mac to triple check.
If all is fine. Use Burn also another free app to burn your DVD. If the end result is fine then the content is not the problem. The encoding used in iDvd to flatten that videos and audio is. Check to make sure you are using the latest version and that your audio and video of your iMovie project playback happily in QuickTime.
I simply suspect that iDVD has come to the end if its shelf life as DVD technology is now dead to Apple.
Hi, just found this as I was searching for a similar problem however seemed to have fixed it myself.
Incase your problem was the same as mine;
I created an iMovie and added various fade in/out sound tracks via iTunes purchased songs. I worked on the imovie for a couple of weeks off and on.
The latest track sounded fine but all the others were up/down fading in and out. Sounded terrible!! After searching the web, I gave up and searched all the iMovie settings and played spot the difference between the latest added 'good' track and the others i previously added.
I found that on the terrible sounding audio tracks I had 'Reduce background noice' ticked. I unticked all these and the sound was perfect. I guess being a novice, I must have ticked this by mistake thinking it was a good thing, but perhaps in hindsight better suited to shot footage with real 'background noise' rather than trying to use it on an audio mp3 song.
Hope it helps anyone making the same mistake as me.