If I understand your question correctly, you have a Media Source with an mp3 that shows up playing in the Audio Mixer (you see it at the volume meter) but you don't hear anything. Do you mean not hearing in the stream/recording or via the speakers of your OBS laptop?
Within the Audio Mixer: click the cog wheel of that source and go to Advanced Audio Properties. Here you'll find the Audio Monitoring dropdown. Monitoring in this context means: listening by the streaming/recording technician or other people participating. It is not obvious to have monitoring enabled since the technician quite often also is the presenter, sitting right next to the OBS laptop with a microphone. Then sound feedback risks are an issue.
At the dropdown you can choose to have the sound going
1. only to your stream/recording: Monitor Off
2. not to your stream/recording but only to your laptop's speakers or headphones: Monitor Only (mute output)
3. to both: Monitor and Output
