like many many others when converting avi to dvd using tmpgenc I suffer from extremely pi$$ poor audio that the default settings produce. For reasons unknown to me I cannot get tmpgenc to run with toolame ;/ .... I get the "invalid audio stream" error ;/ ....I figured this had to be for an obvious reason but after almost 4 days reading and searching with no luck (but finding quite few helpful tips on the way) I gave up!
Anyway I just hashed this method together from various guides, it's tried and tested, and I'm happy with it because it gives the result I was looking for in the first place and with little work (well to me anyway).

I'm assuming you know how to use these tools already, know where to get them, and have a basic knowledge/common sense of ripping and encoding.

tools used

tmpgenc
tmpgenc dvd author
virtualdub
ac3 machine with besweet
nero
*xbmc for help judging audio delay when required

1) get your avi file you want to convert to dvd.

2) use tmpgenc to encode your avi file to dvd format with NO sound*.

use the wizard, or manual settings if you prefer. *if no video only option is shaded out just highlight the line in the audio source box and delete what's in it.

3) open the original avi in virtualdub and extract the wav sound file from the movie.

4) run ac3 machine with besweet on the wav file vitualdub made to make it ac3. change ssrc options to 48000khz and if you want to make an audio delay* do so now.

*I use xbmc to work this out, heheh the xbox does have its uses other than a games machine ;p .. OR you can use tmpgenc dvd author with a source range setting for around a minute say 10 mins into the film where there's a scene with people talking and adjust the audio accordingly to your liking (takes a bit longer but is equally as effective).

5) open tmpgenc and use multiplex in tools to join the files you have.

you may get two mpg files appear so just delete the one at the bottom and import the ac3.

5b) or instead you could just import the mpg and ac3 into tmpgenc dvd author and convert to vobs, I don't see any reason why this wont work but I haven't tried it out as of yet only beacuse dvd author doesn't allow a preview of ac3 and after multiplexing with tmpgenc you can simply check the file is working by using power dvd or any other dvd software.

6) use tmpgencdvd author to author / convert to vobs for burning.

7) use nero dvd mode to complete the process.

..... I hope this helps some of the people who were/are having problems converting their avi's to dvd with tmpgenc and having sound problems!

p.s as I mentioned I did 4 days searching so if this exactly the same as any other method, sorry.