This is difficult to answer without knowing all of the details.  What do these "audio tracks" consist of?  Soundpool objects or live recordings?

The problem is that far too many people create their arrangements backwards!  They "record" vocals and instrumentation before any tempo is assigned to the arrangement.  Then, when they import a file that has an assigned tempo, the whole arrangement is automatically adjusted to the object tempo.  The pre-recorded MIDI file DOES have a tempo assigned to it.

MMM will set the arrangement tempo to the FIRST object (file) imported that has a set tempo assigned to it.  You need the set the arrangement tempo at the very beginning of your project!  Setting the arrangement tempo using the control near the transport controls does no good until you have an object with a set tempo in the arrangement.

There's a few ways to do it:

You can simply import a soundpool object with the desired tempo assigned to it into a new arrangement.  The default tempo for each soundpool object is shown in the media pool window.  Then, all other "tempo assigned" objects imported will automatically adjust to the arrangement tempo.

Or, you can first create a Beatbox 2 track to act as a sort of metronome. You can then make the adjustment to the desired tempo.  Then, all other "tempo assigned" objects imported will automatically adjust to the arrangement tempo.

Otherwise, if using a live recording or an audio object from an outside source, you will need to use the Beat Finder to assign a tempo to that file. It should be resampled to make the beat consistant so it will blend properly with any imported objects.  Then, when you import that file into a new arrangement, the arrangement tempo will be adjusted automatically to the audio object tempo.  Then, all other "tempo assigned" objects imported will automatically adjust to the arrangement tempo.

The best way is to "record" your vocals and instrumentation to the beat of the rhythm track - not the other way around.