I highly doubt there is any significance to such a pronunciation (although I stand to be corrected) but since you ask, here is a technical explanation (based on here and here): There are two types of sounds - ones where you use your voice, like b, d, g, f and z, and ones where you don't like p, t, k, v, and s. The ones that you use your voice for are called voiced and the others are called unvoiced. The sounds "s" and "z" have identical mouth positions (the lips part and the corners pull back while the teeth themselves lightly touch). The difference is whether or not you use your voice when you say them. When a voiced sound comes before an "s", it often makes it sound like a "z". [For example: backs and bags. The "s" sounds like an "s" in backs because there is a "k" in front of it, which is voiceless. In bags, it sounds like a "z" because there is a "g" in front of it, which is voiced.] Since the unvoiced "s" in beis comes after the voiced "b" it can make it sound more like a "z".