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 an 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. Therefore, 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.