Thank you very much @rraud !! I will try to explain it better. Please, if you can have a little patience with me ... I'm from Brazil and I don't know how to speak English very well. But I will try. The situation is as follows: I have three stereo tracks, a music track that is in the WAV format, a track with a male voice in WAV format and a track with a female voice in WAV format. So, I opened the multitrack mode in AS 14, opened 6 lines of channels and put the three tracks to edit and format as if it were a single soundtrack with the song we are singing. When I went to convert this multitrack to be able to save the project in MP3 format, I realized that in the SA14 Processes menu there is no channel converter tool. And I can't convert that multitrack. Do you want to know anything else?

Your really need a multi-track DAW application to mix down your dialog and music tracks... (Vegas, Reaper, ProTools, Audition, ect. Audacity is a free DAW that would work.

SF is an multi-channel application for mono, stereo and 'surround' type files There are some work arounds with SF, but there are a lot of issues that causes many difficulties and compromises. If you can manage to line up the files the way you want and adjust the separate channel volumes and cross-fades, You could 'Save as' to a mono or stereo file (MP3 or otherwise). which would down mix the elements.

As I previously stated, I am not familiar with AS (Audio Studio, so maybe someone with AS experience can elaborate on my generalization based on SF Pro.