I found this in mod_conference.c:

/* Set MOH from variable if not set */

if (zstr(conference->moh_sound)) {

conference->moh_sound = switch_core_strdup(conference->pool, switch_channel_get_variable(channel, "conference_moh_sound"));

}

This evaluate channel variable conference_moh_sound only if the "conference" command creates a new conference. Give it a try. ;-)

Greetings,

Roland