If you wanted to, you could change the font. Not too much, but just a little bit. Readers notice, but it won't stand out or break the rhythm of the story.
Okaaaay then. DON'T change the font. Ignore me. I'm just a noob.
I wouldn't change the font, color, or make it bold. It'd look pretty unprofessional imo. You could use italics and assign a name or quality to each voice while keeping proper dialogue punctuation.
No need to reinvent the wheel... or design it incorrectly.
Screen readers indicate the use of italics to the user?