Final answer:
The sentence should include commas around the clause 'in case you're wondering' to properly set it off as a non-restrictive clause. The decision on the use of commas can depend on the intended tone of the sentence.
Step-by-step explanation:
The sentence "Max Mayfield, in case you're wondering, knows how to play the harmonica." should include commas to set off the non-restrictive clause 'in case you're wondering'. This clause does not change the meaning of the main clause, which is that Max Mayfield knows how to play the harmonica. However, the commas are not necessary if the intent is to directly address the reader or listener, as in a conversational aside; in that case, the sentence would be "Max Mayfield in case you're wondering knows how to play the harmonica." The decision on commas often depends on the intended tone and flow of the sentence.