No sound output from JBL speakers

I have a MacBook Pro 13 inch - 2 years old.

I have just bought some speakers called JBL Pebbles. These are USB-powered.

When connected directly to the USB port on my 2015 MacBook Pro 15 inch they work great.

BUT ... when I connect to my newer MacBook Pro - of course, using the Apple dongle because there are no ports on new Macs - the speaker is not recognized.

I checked System Prefs - Sound - Sound Out. Nothing.

I thought the new Macs with a USB dongle were equivalent to a regular USB port.

Any ideas what to try next? Thanks.