Unfortunately, the front-facing stereo speakers of past Moto X models are gone, now replaced with a single speaker up top. However, the Moto Z offers the option to add much larger speakers on the back thanks to the JBL SoundBoost Mod.
It's a bulkier Mod than the Power Pack, but the SoundBoost packs a solid punch for the still relatively compact size: they're each 3W speakers with output ranging between 200Hz and 20kHz. The SoundBoost doesn't produce massive sound, but it also doesn't seem canned or overly contained. It'll do the trick for a beach party or backyard, in other words.
The built-in kickstand is a nice touch, letting you prop up the phone while the speakers are in use – that way, you don't have to lay your screen flat on a surface, or prop the Moto Z up against a wall. And the 1,000mAh battery within means the SoundBoost won't start sucking up your phone's internal battery for upwards of 10 hours of playback.
For RM599, it's a fair offering, albeit not one we'd want to keep snapped on at all times due to the extra bulk. And that brings us to the next question: in that case, why not just go for a more powerful Bluetooth speaker that can connect to any wireless devices that you or your friends have, rather than limiting it to the Moto Z?
In fact, JBL has some strong portable speaker options (like the Charge 2 or 3), and while you might spend a little more than the RM599 here for something good, you'll probably get higher-end hardware in the process alongside wider device compatibility. It's your call, but give some thought to the lifespan of the speaker you're considering.