exten => 2,1,SetVar(DIR=/tmp/)
exten => 2,n,SetVar(NR=1)
exten => 2,n(play),playback(${DIR}${NR}.gsm)

Executing SetVar("xxx", "DIR=/tmp/") in new stack
Executing SetVar("xxx", "NR=1") in new stack
Executing Playback("xxx", "/tmp/1.gsm") in new stack
file.c: File /tmp/1.gsm does not exist in any format
file.c: Unable to open /tmp/1.gsm (format ulaw): No such file or directory
app_playback.c: ast_streamfile failed on xxx for /tmp/1.gsm


l -l /tmp/1.gsm 
-rw-r--r-- 1 root root 1584 2007-05-27 21:27 /tmp/1.gsm