For the love of the keyboard you already own.
Keyclick is a Mac OS X preference pane that provides adjustable audio feedback for keystrokes, mouse clicks, and the scroll wheel. Years ago keyboards were made with premium key switches that provided a resounding click when a key snapped over. Today most keyboards are made using rubber domes which can provide a Similar force Displacement or tactile feel, but not the sound. Human factors work on video Terminals in the 1980s showed that the perception of keyboard crispness is correlated with audio feedback. Keyclick provides that feedback while improving on the original.
While there are existing programs that imitate the sound of an old fashioned typewriter, they do not appear to be intended as a practical touch typing aid. To be most effective, the feedback needs to be subtle and non-distracting. Keyclick allows you to adjust the volume up or down at any time without even visiting the Keyclick Preferences Panel. Use Cmd-Shift-) for louder, and Cmd-Shift-( for softer (or define your own ShortCuts). The volume is also affected by the system volume setting so it can ADAPT more easily to quiet or noisy surroundings. I suggest starting just above the softest volume that's clearly discernible.