The Collaborative International Dictionary of English v.0.48:

Betacism \Be"ta*cism\, Betacismus \Be`ta*cis"mus\, n.
   Excessive or extended use of the b sound in speech, due to
   conversion of other sounds into it, as through inability to
   distinguish them from b, or because of difficulty in
   pronouncing them.
   [Webster 1913 Suppl.]