Hello there Andrew Halliday,
It sounds like you are connecting external speakers to your MacBook Pro but no sound comes out of them. I would first, and I know this sounds corny, make sure the volume is turned up:
Turn the volume up or down
To change your computer’s volume, click the Volume control in the menu bar, then drag the slider to adjust the volume.
If the Volume control isn’t in the menu bar, choose Apple menu > System Preferences, then click Sound. Click Output, then select the “Show volume in menu bar” checkbox.
If it is, next I would reset the PRAM or NVRAM on your computer with these steps and then retest the issue:
How to Reset NVRAM on your Mac
- Shut down your Mac.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
- Turn on your Mac.
- Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
- Hold these keys until the computer restarts and you hear the startup sound for a second time.
- Release the keys.
After resetting NVRAM, you may need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information.
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