The volume is quiet for some voices and LOUD for others in the Memrise official German decks (at least German 1, German 2, and German 3).
If you turn it up to hear the quiet voices you get BLASTED by the loud voices. Usually, it’s the “learn with locals” who are too quiet and the invisible Memrise voice that repeats what they say that is too quiet.
To fix this all audio files should be “normalized”; ie, set to the same volume. It’s not rocket science. Simply import the audio into any editor like Audacity and you’ll immediately see the audio gain is WAY different.
I’ve written to Memrise about this using the “contact us” feature, but there’s been no change. I’ve seen others write about this as well in the forum. As another person pointed out, you don’t want to solve this by keeping the volume loud because you’ll disturb your neighbors – and some of us don’t want to wear headphones all the time. Please, please do something about this!