Ugh... Voicing. I'm an uncertified terp right now (although, I passed the new RID-NAD Written Test of Knowledge 1 yr ago... YAY!). I've been working as a provisional interpreter for the past year through a local agency. Now, to answer your question...
Many d/Deaf clients that I have had the pleasure of interpreting for use their voices. So, I normally only sign during an assignment.
During my interpreting training program, we only have 1 SIgn-to-Voice class. Definitely not enough practice before getting out into the "Real World".
Stage Fright. Yes, yes, and yes. It is difficult (for me at least) to continuously speak over a client's signs. Processing their information and putting it out in voice, while STILL taking in the message from the Deaf consumer is difficult.
As my dad always tells me when I feel like I did a bad job, "If interpreting were easy, everyone would be an interpreter."
So true.