Operetta-singing mice on a pirate ship: Where have I seen that before? Well, MIGHTY MOUSE AND THE PIRATES for one. Paul Terry liked to recycle his plot ideas and this is about the oldest version from his studio that I have seen -- Mighty Mouse is replaced by the Navy Air Force.
Although there are lots of the usual budget-stretching Paul Terry cheats throughout, including a recycling first shot, the people who did the art certainly took their time. The characters are as highly detailed as any I have ever seen and the pirate ship is also fully realized. If the gags are recycled from a hundred earlier cartoons and will appear in a hundred more, they are appropriate, so this one is quite watchable.
Apparently it ran short as there is a one-minute epilogue. Even with that, this one times in at less than six minutes -- but I saw a Castle re-release, so it may be trimmed.