I got very first version (proof of concept) working.
This is ugly hack using debian's package, but it shows mumble is possible on N900.

Instructions: (I think this is safe, but you make your decisions)

step 1) Install some dependencies
step 2) Download missing libraries from debian armel, do not install them, but open them and put actual library files in one directory.
This is list of needed:
libstdc++6_4.4.3-1_armel.deb
libg15daemon-client1_1.9.5.3-8_armel.deb
libg15render1_1.3.0~svn316-2.1_armel.deb
libg15-1_1.2.7-2_armel.deb
libprotobuf5_2.2.0a-0.1_armel.deb

step 3) Download mumble from debian, take only binary.
mumble_1.2.1-3_armel.deb

And then start with
LD_LIBRARY_PATH=. ./mumble


If you are lazy, and just only test is this working or not you can use my tarball (do step 1 still):

When you first time start it you can use wizard to test microphone and speakers. All these windows are too big to fit on screen. Just skip or start PC version of Mumble same time and read from there.

What works:
Connecting to server
speaking (another participant can hear you)
listening (what another is speaking)
sending text message
receiving text message

What is not working:
Speech-to-text
Audio Wizard (doesn't fit the screen)