[SOLVED] Very low voice volume

Hi everyone,
I'm new to MuseScore. I've installed it to open a midi file and extract mp3 from it, one for each voice. I have piano, alto, tenor. The first extraction was good; the default voices are not great but I could hear them clearly, even if just a little low. Here's how Altos appear in the mixer:

https://i.imgur.com/NILHPlk.png

Then I installed the Muse Choir addon from Muse Hub 1. Now the voices in MuseScore have a really low volume:

https://i.imgur.com/dmXMX97.png

So as a beginner I don't understand how just changing the voice pack can produce so different results, if I haven't touched anything else. Can you help me make the Muse Choir addon work better?

Thanks!