In revision 16249 I am seeing an intermittent problem as follows:

1) conference is setup to play an alone sound and moh 
2) first person calls into the conference 
3) very intermittently the "alone" sound if not heard. Instead moh is played immediately

I have tracked this down to a race condition in mod_conference where the line 1011 within 
conference_thread_run() checks the value of conference->async_fnode at the same time, on 
a separate thread, conference_add_member() is updating that value via conference_play_file.

As a result, intermittently, the alone sound is enqueued but then immediately "over written" 
by the moh found.

A possible solution would be to surround the if statement at 1011 with

switch_mutex_lock(conference->mutex) 
… 
switch_mutex_unlock(conference->mutex)

Let me know if I should go ahead and enter a Jira and post a patch.

Peter