Can't play g711a files

I have my asterisk box running on a CentOS server that was compiled from source. I have some g711a files that I want to use instead of the default alaw files that came with the installation. When I made the dialplan for the files they wouldn’t work. the PBX just hangs up the phone (or skips to the echotest) before a sound file can be played. The server plays the alaw and ulaw files fine but not the g711a files. Here is what my iax2 phones looks like in iax.conf

type = friend
host = dynamic
careinvite = no
dtmfmode = rfc2833
allow = ulaw
allow = alaw
allow = g711a
transport = udp
nat = yes
callerid = User One <1001>
secret = 1001
mailbox = 1001@default

I don’t understand why this isn’t working. What do I need to install in order for these files to start playing. I have the dialplan for the files looking like this

exten => 1,1,Answer()
exten => 1,n,Playback(/etc/asterisk/sounds/demo-congrats)
exten => 1,n,Hangup()

I would have thought that g711a was the A-Law variant of G.711, i.e. what Asterisk calls alaw!

What can I do to make them play when I user the playback application

If they really alaw, rename them to .alaw, otherwise convert them to a supported format. Remember that you do not include hte file type in Playback.