How to say 'Piezo'
Many of the pickups we use on upright bass use a piezo-electric element to produce sound. A piezo-electric element is a means for creating an electrical signal, usually by combining a crystalline substance with a ceramic, and connecting wires to capture the electrical reaction which occurs when pressure is applied.
Most of us shorten "Piezo-electric" to "Piezo," for the sake of convenience. But a lot of us say it wrong; some unintentionally reverse sounds (PEE zee Oh) and some make a long "I" sound (PIE zoh). Some people make a soft "z" sound (like "zing") and some people make it a hard "z" (like pizza or pizzicato). I've heard plenty of other permutations, too.
Most dictionaries agree, however, that the correct pronunciation (in the USA, at least) is:
pee AY zo
So, long "E", accenting the second syllable with a long "A" sound, and a soft "Z" with a long "O" sound.
Here's a recording of what it should sound like.