Q: Je ne parviens pas à connecter un haut-parleur à mon mac mini.

Google translate:

It's been days that I tent to connect a speaker to my mac mini.

My mini mac OS X uses the Yosemite System version 10 10.2. I bought it a month ago.

The speaker is a "JBL Micro Wireless". It can connect via cable or via Bluetooth.

Thank you to answer me asap

For bluetooth connection did you successfully pair the spears to the Mac?

When you try to pair does the Mac see the speakers?

The follow is applicable eventhough it says Mavericks

OS X Mavericks: Set up Bluetooth audio devices

For wired connection, make sure you are plugging it into the headphone jack.

For both methods go to System Preferences>Sounds>output and make sure the device is selected as the output device.

Several threads on this subject although not to your particular wireless speakers.

For example

Bluetooth problem with OSX Yosemite

See some suggestions here:

Wi-Fi and Bluetooth: Potential sources of wireless interference - Apple Support

And these options via Apple mentioned on some of the threads:

Intel-based Macs: Resetting the System Management Controller (SMC) - Apple Support

How to Reset NVRAM on your Mac - Apple Support