Possibly via the application NirCmd. NirCmd is a small command line programme that includes various system commands. These commands can also be integrated into the stream deck.
I tried the following with it:
Integrated the command "System / Open" in the stream deck and stored the following command there (without inverted commas):
"C:\Windows\nircmd.exe changeappvolume Firefox.exe -0.5".
This halves the volume of Firefox.