As a rule it is good practice to wear headphones when metal detecting, however external speakers can be a great advantage in certain circumstances.

Sometimes it is a good idea to hear your surroundings while metal detecting in the forest or on a beach, these are just 2 advantages that spring to mind

Metal detectors very often have speakers, most speakers are still not loud enough for noisy environments such as detecting near to the surf.
I find an external speaker is an advantage when filming because the camera can pick up the signal from the metal detector.

My external speaker set up

When using the XP Deus 2 or XP Deus 1 in the Master mode,

The Master Mode is the Deus 1 WS4 or the Deus 2 WS6 mounted on the stem, replacing the remote control.
In the master mode, there is no speaker option, only wireless headphones.

My preferred method

This is how I connect an external speaker to my WS6 Master.

Firstly you will need an additional set of wireless headphones the cheaper XP compatible versions are the XP WS Audio or the XP WSAII, these are ideal because they have a removable puck.
Once removed from the headphone band they can then accept a horseshoe adapter into that is a 3.5mm jack plug leading to my external speaker.
I use a speaker called an Aporo, it's reasonably cheap and has a rechargeable battery.

External speaker showing the volume control

Speakers for your metal detector

Once connected, the volume can be adjusted and the speaker can be mounted anywhere you wish.
One thing worth noting is the audio quality, you will never get the same audio experience with an external speaker.
The genuine XP headphones will always sound much better.

You can see the speaker in action here on the Skill School channel.