I just came home. Here is a way to fix it:
Go via SSH (for Example PuTTY) into your Raspberry. Or hook up a monitor and a keyboard to your Raspberry.
Login and password are both “volumio”.
On the command line type: “cat /proc/asound/cards”
This will show you your soundcards. Look for [ODACrevB ] and notice the number which stands directly in front of the [ODACrevB ]. Mine for example is number 5!
Then type “alsamixer -D hw:5” where you put your noticed number after the hw: .
You will see a graphic with 2 Lines: PCM and Softmaster. You can go with your cursers (left right) to select PCM and then go with curser (up down) to select the maximum level (100).
You can leave with escape and it should be done.
All of my machines (see signature) are remembering this setting after reboot.
Hope it helps.