Converting a Musical Score to a Playable Melody?

SA_Democrat asks: "As a geek who has recently discovered that he has a voice, I find myself looking for a particular style of software. I've joined a local chorale group, and am often the only bass singer in attendance. This means that I have to puzzle out fairly complicated pieces of music and pick out the melody on a keyboard between rehearsals. As a person who decodes music rather than someone who sight-reads, I find this extraordinarily difficult, especially when managing differing key and time signatures within a given piece. Does anyone have any experience with open-source software that allows the user to enter a piece of music using musical notation, and then plays that piece? I have found an astonishing array of programs that will play MP3, WAV files etc. but have not located anything that uses this more old fashioned method. If possible, the software should understand common notation like time signatures, keys, glissades, and so forth. What does Slashdot recommend?"