Multi volume

Players like WMP don't support multi volume discs.

MP3Tag offer various options to renumber the tracks.

If you have no numbering at all, look up the album in a online database.

Consecutive numbering

Select all the directories by selecting a top level one

If needed sort by album title or directory.

Select the Auto numbering Wizard

 

Now you have a consecutive numbering

 

Preserving the disc number

Select a single directory and start with a value like 101 for disc 1, 201 for disc 2, etc.

 

After combining the albums you have them sorted by disc/track.

Preserving the disc number and the original track number

I have all my classical music organized by work. So if there is a problem with a track or it is simply missing, it is convenient to know its original track number

 

Add a disc number to all the tracks. This is the DISCNUMBER track in IDV3.

 

Choose Convert > Action and define an action, call it DiscSet

Choose Format value as the type of action.
The field the action is applied to is the track
The action is:
$if($less(%track%,100),$add($mul(%discnumber%,100),%track%),%track%)

$mul(%discnumber%,100) - multiply the discnumber by 100
$add($mul(%discnumber%,100),%track%) - add the track number
$if($less(%track%,100),$add($mul(%discnumber%,100),%track%),%track%) do this if the track is below 100 only.

 

Select all the tracks

Apply the action

Now you combine them in one album you have them ordered by CD and within a CD by track while preserving the original track number.