Tremolo doesn't play

Tremolo playback simply is not supported in the current release. It will be in 2.0. Feel free to try it out in a beta or nightly build.

@ rdr... Welcome aboard.

Re: MuseScore 1.3...
When it comes to playback features, (especially concerning 'nuances' like tremolo, trills, etc.) an oft repeated answer is that MuseScore is primarily a score writing application, and that playback is of secondary importance - sort of like a 'spell check' for mistakes.

That being said, it is to the credit of the developers - who made MuseScore 1.3 so much fun and feature rich - that the software almost encourages users to push the playback envelope.
So, for your consideration, and using 1.3, I have cobbled together an attachment which plays tremolo. Please have a listen.
(Also, if you click on menu item: Display, and then on 'Show Invisible', you can see the hidden notes which allow for tremolo playback.
This is merely my quick example.There are other people who can work even greater magic...
MuseScore is one heck of an app.

So, welcome to one of the best support forums on the internet; and, as already mentioned, tremolo will be supported in 2.0. It is already available in the betas and in the nightly builds:
http://musescore.org/en/node/42051
and
http://musescore.org/en/handbook/comparison-stable-prerelease-and-night…

We all anxiously await 2.0's 'official' release in the foreseeable future. Then we'll have a go at a new set of bells and whistles.

Regards.