While I like the answers provided, there is a context in which that doesn't work - fast paced conversation. Having one or more people signing and others using English to have an argument sounds like fun, but for normal conversation you can skip a few 'Harold said' by virtue of the fact that there are only two people in the conversation. If you have a similar scenario - one person who argues in English (we assume the mute person can either hear or lipread) and the second signing, then you can always use italics to indicate the signed dialog.
I've seen this used as a mechanism for non-vocalised thought;
"I don't like this situation at all, Pedro!" I'm going to get killed on this intersection; I just know it. Harold was starting to lose faith in his spirit guide...
Now, if you can use it for non-vocalised thought, it just seems that it would make for a great solution to a rapid-fire conversation including signage...
"No, Pedro! We can't just cross a busy street with our eyes shut!"
Of course we can - where's your sense of adventure?
"Back in the field we landed in after jumping out of that plane, just like my lunch!"
You know, I'm starting to lose faith in you as a spirit sidekick...
It still reads well, is fast paced, and has the added benefit that you don't have to throw in the occasional 'Harold said' so the reader can keep the pace of who's saying what.
[]. So +1, because this is what my answer would have been. – F1Krazy Dec 06 '19 at 12:17