All four of the previous 'Scooby Doo: Mystery Incorporated' episodes are very good, they were on first watch and were also on rewatches with more to appreciate each time (that is including the animation style). My first watch opinion of "Song of Mystery" was extremely positive, didn't like the character writing for one character, which was a general problem in the early episodes, but absolutely adored the atmosphere which the show delivered so well in consistently.
While not one of the best episodes of 'Scooby Doo: Mystery Incorporated', "Song of Mystery" is still very, very good and continues the high standard that the previous four episodes set. A standard that thankfully was consistently held pretty much throughout the show's run. The usual strengths that were present in all four of the previous episodes are present here, as well as some nice character moments, though there was a little more potential for it to be even greater than it turned out considering how good the premise was.
So much is good here in "Song of Mystery". The animation is rich in dark atmosphere and also rich in attention to detail. The music is always harmonious with the action and the theme music is one of the best of the franchise, love the mix of spooky horror and spy thriller in the way it's scored and how the credits are designed. The writing is a lot of fun while also being thought probing, it never forgets to be fun while also scoring on the more dramatic parts.
There are a few nice character moments of the amusing kind, like with the costumes and Fred's line to Daphne about not being too close. The story is always compelling and suspenseful with a completely unexpected reveal (one of the more surprising ones of the first season). The villain is wonderfully creepy. The voice acting is very good all round. Most of the characters are well written, especially Shaggy and Scooby who are pretty much the Shaggy and Scooby fans of the franchise know and adore.
Excepting Velma, really do not like how bossy she is in this episode and the first half of the first season in particular overdid her bossiness to a jerk-ish degree.
Concluding, very good once again. 8/10.