If you are using ABC2Win you type the tune alphabetically, then goto Draw command and sheet music version is displayed. Goto Play option and it does just that. Whlst playing you can interrupt to change tempo. Pretty neat. Drawback is that sound output is from system squeaker only.

Download ABCPlay from the homepage and it'll play through the stereo output on different instruments etc. Drawback is it won't display / print sheet music.

It's all free at the ABC homepage.