I work full-time (40 hours/week) in a business setting, and I would say about 50% of my time is voicing. Most of the time this is one-on-one interactions with the occasional staff meeting in which the deaf clients participate fully. How much voicing a terp does, though, is going to depend a lot on what setting they are in. Educational terps voice much less than business terps, in my experience. (But that, too, depends on the students.)
Ayala, I think I know the answer to your question, and it's a very simple one. This is one thing I remember hearing in the brief time I spent in an ITP.
The reason many terps shy away from voicing is because it is always easier to go from your native language into your second language. This is true of voice interpreters too - it's just a fact of interpreting, the way linguistics naturally works. Additionally, because no two people sign identically, a terp has to adjust for "accent" (manual differences in signing) every time they have a voicing job.
So it starts as a native language thing, and then it's compounded by fear. Getting over the fear leaves only the "second language to native language" hurdle to be conquered, and that comes with practice.
