Looks like all Audio tracks that have a volume set other than 0 dB in a Resolume 5 composition, get the minimum volume level set to -40 dB
and when you open this composition in Resolume 6, you can't make the clip volume lower than -40.
I created a ticket. #11026
In the meantime if you drop the same file to the clip slot, the ranges should reset, while keeping other settings.