PAL to NTSC Conversion Issues

Hi All,

I have been trying to convert some of my old PAL VHS tapes to NTSC  
DVDs before they completely die. I have a DVD recorder which can  
create a PAL DVD from the PAL VHS input signal, which I can then bring  
into the computer using vobcopy.

I then use the following transcode command to generate separate m2V  
and ac3 video and audio streams in NTSC format:

transcode -i tmp/movie.vob -y mpeg2enc,raw -x vob -D -2 -Q 5,5 -F 8 -Z  
720x480 -n 0x2000 -E 48000,16,2 -b 256 --export_asr=2 -o tmp/movie -m  
tmp/movie.ac3 -J
smartdeinter=diffmode=2:cubic=1:blend=1,modfps --export_fps 29.970,4

This works OK, and then I use mplex to recombine the video and audio streams.
mplex -f 8 -o tmp/final.mpg tmp/shrinked.m2v tmp/movie.ac3

The resulting final.mpg file is playable on the computer, I can  
successfully create the xml file for the DVD writer using the makexml  
command,
makexml tmp/final.mpg -dvd -overwrite -chapters 5 -out tmp/movie but  
when I try to make the DVD using
makedvd "tmp/movie.xml"
I get a succesion of messages like the ones below
WARN: Discontinuity of 4320 in audio channel 8; please remultiplex input.
WARN: Previous sector: 94.021 - 94.021
WARN: Current sector: 94.069 - 94.069
WARN: Discontinuity of 6480 in audio channel 8; please remultiplex input.
WARN: Previous sector: 94.069 - 94.069
WARN: Current sector: 94.141 - 94.141

The resulting DVD is playable, with decent video, but no sound.

Is there anything in the transcode or mplex commands that I could  
change to avoid the makedvd errors?

Thanks

Jeff

PS I do know that there are US spec DVD players that can play PAL DVDs  
on an NTSC TV. I have one myself, but I'd like to convert the DVD so  
that special players are not required.