ive written a few semi-mute characters - that is, they rarely talked, only if they had no other option - be it because theyre autistic (like me), or deaf, or both. Typically, I'd have sign language incorporated in, with speech bubbles coming from the hands - if its assumed that the cast can all sign - otherwise, I'd have another character translating.
on one hand, i love mute characters that use telepathy or some other creative method to communicate, but on the other, i crave realistic portrayals of mute characters. thats why i veer towards using sign language, even when magic and superpowers exist in a universe - its what we use in real life. i think a good compromise is, say, someone signs what they want to say, and its made into speech by a magic spell or the like. something that still incorporates signing into the equation.
also, of course, you can always utilise the fact that youre showing an image - body language, facial expressions, there are tonnes of tools of expression that are nonverbal.