Under the advanced tab of the OGM properties menu, one is "Play external Wav/mp3 file". If you check this and do not load a message on the message tab, then it will play a wav file. You can include variables in the file path to the audio file. For example you can have it play the following file:
C:\IVMmessages\%currentmessage%.wav
You can change the "currentmessage" variable value to be the name of the message you want to play. So if you have sound files called chicken.wav and rooster.wav, then in a previous OGM you could run the genfunc plugin and assign the value of chicken to the currentmessage variable, so that when it gets to this OGM, it will play the following file:
C:\IVMmessages\chicken.wav
Hope this answers your question.