asterisk users - Jun 2010 - debug message: Internal timing is disabled

Hi all,
i got a lot of this messages if only one caller is in a meetme
conference and it playing a MusicOnHold Sound. If a second Caller
entry the Conference the messages ended.

DEBUG[11794] channel.c: Internal timing is disabled
(option_internal_timing=0 chan->timingfd=61

What does this message mean?

Thanx for answers
Daniel