To bring us back closer to the current strip, I'm not sure it has been pointed out explicitly but according to
this comic, Suggestion has no somatic component so it doesn't matter that Belkar can't hear. I thought it didn't matter if the target heard any casting but this way it is spelled out, so to speak.
I'm pretty sure the use of spell names when casting is more so the reader knows what is going on rather than representing what others would hear.