Trace the letters back to the pins. The inside of the case should have two plastic sheets (one on top of the other).
One plastic sheet will have printed conductive tracings that go to one set of pins on the circuit board and the other sheet will have tracings that go to another set of pins on the circuit board. When you press down on a key, the tracings on both sheets will touch each other. This completes the circuit and tells the board to send a letter, number or command back to the computer.
What you need to do is to label each conductive circle on both sheets with the letter key that corresponds to it. So, for instance, if you were to press down "A" on the keyboard, label the plastic circle on both sheets of plastic that would be pushed together if the "A" key would strike (see picture of labeled sheets). Download Soundplant and links. Soundplant is offered as a free download which includes a trial of the registered features: Select older versions of Soundplant are still available if you need them for older computers: Here are some example .keymaps to play with: beat pad 2 [4.5 MB] - Drum pad of sounds extracted from various breakbeats, with a mix of well-worn classic hip-hop sample sources and dug-up finds.
Updated with effects and a few new loops, compatible with Soundplant v.40 and up. sine wave church organ [0.008 MB] - A chromatically tuned, heavily reverberating organ made from a simple sine tone, utilizing 'fade/hold' keymode sustain the tone while holding down keys and slowly fade out on release. Siebdruck, wie gemacht? Tips & Hinweise.