After getting annoyed again trying to hear the Google Assistant I realised that I could set up a Bixby routine to turn up the media volume automatically while the Assistant is running.
This is working splendidly and is a great solution to the problem of having no separate volume control for Google Assistant.
Step 1,
Go to Advanced Features, Bixby routines and select add routine to start making a new routine.
Step 2,
Select If, then Event and App opened.
Step 3,
Select Google and press Done.
Step 4,
Select Then and Sounds and Vibrations then Media Volume, set the volume you want for the Google Assistant and press done.
Leave when routine ends on reverse actions and press next ,give the routine a name (I left it on Google) choose a icon and colour pressDone.
That's it hope this helps anybody having this annoying problem.