Haven't tried changing the park call yet.
It's not in the mix.pl script that you can generate a single wav file. The mixing scripts are responsible for mixing the wav files (as the name implies) into a single wav, gsm, mp3 or ogg file.
This might help:
Here's the entry in extensions.conf for the recordings:
; this is used for recording conference calls, the client app sends the filename
; value as a callerID recordings go to /var/spool/asterisk/monitor (WAV)
; Recording is limited to 1 hour, to make longer, just change the Wait,3600
exten => 8309,1,Answer
exten => 8309,2,Monitor(wav,${CALLERID(name)})
exten => 8309,3,Wait,3600
exten => 8309,4,Hangup
; this is used for recording conference calls, the client app sends the filename
; value as a callerID recordings go to /var/spool/asterisk/monitor (GSM)
; Recording is limited to 1 hour, to make longer, just change the Wait,3600
exten => 8310,1,Answer
exten => 8310,2,Monitor(gsm,${CALLERID(name)})
exten => 8310,3,Wait,3600
exten => 8310,4,Hangup
You can try using Mixmonitor instead of Monitor. It has it's advantages and disadvantages. It only generates a single wav or gsm file.