When you're burning an MP4 file on a DVD+RW, you also have to make sure that the burning mode is correctly specified. Some DVD players might not have support for the encoding mode you have selected, therefore audio is cut completely.
To play the file on a DVD Player, burn the MP4 video on a new DVD Video compilation. Alternatively, modify the output sound codec to match the one specified on the manual of the DVD Player (LAME codec or AAC).