Due to some prerequisite requirements, it is possible that mp3 files will not work with certain streaming software. The best solution is to convert all sound files to the format "OGG". 

When you upload a new file to Streamlabs it should convert the file to an OGG after it has completed. 

If you would prefer to have it an OGG prior to uploading you can also try using a converter site like: https://convertio.co then upload it without any conversion done through Streamlabs.

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FreemacOS 10.14+309MB