set file format by batch / powershell

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Can anyone point me in the right direction on this one...

we have some software installed, which is web based. it's call recording software, to play back the recording you have to install the call player, which is fine and works, but when you log off and back on, it loses the software default. the audio files download ot play in *.opl format. The software vendor have said it's not an issue on their end and we just need to set the default application for the file format to .opl

The software installed to %appdata%/roaming/vendor/package.... etc

Windows default application doesn't have the file format listed, so... can I create a batch file / powershell so that when the user logs on, it puts the default to the application path. The application path never changes.