Separate mixer adjustments for separate parts: no longer possible in MS3

I use MS mainly to transcribe scores for a SATB choir to produce pdf and audio files (MIDI and MP3).
I usually create audio files for each part (one file for sopranos, one for altos, and so on). In each of these files, the part is played together with the other parts, but at a much louder volume.

To get this working, this is the procedure I follow in MS2:
1) create 4 parts using the "New all" button of the "Parts" window
2) add all the other instruments to each of the parts created in 1)
3) open the Mixer
4) click on a tab of a specific part and adjust the volume as desired on the Mixer. For example, click on Soprano part tab, increase the volume of Soprano and reduce volume of Alto, Tenor and Bass.
5) click on another part tab and modify similarly as to 4). For example, click on Alto part tab, increase the volume of Alto and reduce volume of Soprano, Tenor and Bass.
6) do the same for all parts,
7) Create separate audio files via File\Export parts..

With this procedure, I obtain a specific setting of Mixer for each individual part (and these settings are saved when the file is saved).

Unfortunately, I am not able to replicate the procedure in MS3. The mixer settings are always the same for all the parts and it is not apparently possible to get different settings for the different parts.