In the Logic app:
Type: Number
Value: +0.1
If I press the + on the remote the value disappears.
The flow seems to be right. You may have found a bug. I recommend reporting this to Athom. Perhaps someone else has another suggestion?
As far as your variable is concerned: Number is the right type. Homey will automatically adjust the +0.1 to 0.1. Your flow corrects that when it runs for the first time to the current volume + 0.1, so you should be fine.
Thanks for your reply.
Have a great weekend 
Remove any spaces that are in the calculation, including the ones around the tag.
@Caseda
I wish I could show my gratitude for this suggestion.
It work perfectly now.
You are the the man, you saved my day.
I did not just spend hours to figure this out, but days of restarting Homey, deleting app and adding devices.
If you ever come to Copenhagen I WILL buy you a beer.
A space created all my problems.
Thanks again 
Why didnāt I think of that! Nice one!
You tried to help and I appresiate that.
I have spend so much time and the frustration took over.
Thanks to both of you.
Is there a trick to increase the lvl by 0.05?
It seems that Sonos only accepts 0.1 steps.
A little late but sonos can be adjusted by percent and if you update your variable as variable + 0.01 it will be like 1% up every press.
