/β/ – Voiced Bilabial Fricative-Approximant

In Spanish, both the /b/ and /v/ sounds are usually softened. Spanish speakers do this by touching their lips together for just a brief moment and holding the sound for much less time. In fact, sometimes the lips do not touch each other at all, resulting in a sound like /w/. 

As a result, many Spanish speakers will often replace /v/ with /b/. For instance, they may pronounce the word "vaca" as "baca." You will notice that this difference is much slighter than it would be in English.