When it says this? On starting, or connecting to server or when another client is on the server? Are you starting from terminal, or is there some pop-up warning?

This is what I get:
~ $ mumble.sh
Unsupported date format character at 1 (%-d.%-m.)
QGtkStyle was unable to detect the current GTK+ theme.
CELT bitstream 8000000b from libcelt0.so.0.0.0
Locale is fi_FI
...

Do you have this: /usr/lib/libcelt0.so.0

On repository there are version libcelt 0.7.1-2
Mumble has bundled 0.7.0 (which is dropped when optified)

If it is necessary, I can try to take that 0.7.0 version in, but it misses whole point of shared libraries.

CELT 0.7.0 and 0.7.1 should be compatible
http://www.celt-codec.org/
(but 0.6.X is not)