Conduit would only be important if you think something is likely to hit it. What is important, conduit or not, is to use wiring designed for wet/outdoor locations. It WILL get wet.

Online it just talks about how it can decay underground after long period of time.

Online it just talks about how it can decay underground after long period of time.

Online what?

Any wire that's not approved for wet areas can indeed suffer if used underground. Putting it in conduit will not help. Underground conduit is ALWAYS a wet area (believe me, I've watched water flow out of high voltage power conduits for ten minutes after we cut them open.

Further, wire not approved for UV exposure will degrade if exposed to sunlight.

DavidR, are you proposing battery-powered speakers? If not, wireless speakers don't exist!

Yes, of course - rechargeable Bluetooth speakers. Put the speakers where the people are.

I guess I'm late to the discussion, morganlp21, Are you still working on this?

I would mostly agree with DavidR. Having multiple speakers spread out around the listening area gives the best overall effect. Think of a department store. They'll have hundreds of speakers throughout their space but each playing softly. You can hear the music everywhere but it doesn't drown out anybody's conversations.

I also agree that the speakers should all be playing in mono. Stereo relies on the listener being in a "sweet spot." That's impossible over a large area. Stereo is pointless and annoying under these conditions.

Where I don't agree is the world's obsession with wireless speakers. (Not dissing DavidR here, his is a valid consideration, it's just that in every discussion, everywhere, about speaker placement these days, wireless is the first "solution" everybody jumps to.)

Wireless doesn't mean electricity-less. You either have to plug them in to power them, plug them in to recharge them, or change batteries all the time. Plugging in defeats the whole point of wireless. Everybody I've seen go the wireless route ends up schlepping the speakers out every time the want to hear music and schlepping them back inside afterwards. And they all still have the problem of single point sound being too loud for the people nearby and inaudible for people across the yard. None has opted to continually schlepp multiple speakers outside.

Contrast that with a wired system where you just have to flick a switch or two. Guess which will get used the most ?

Wiring is pretty easy. Low-voltage landscape lighting wire is made for direct ground burial and works perfectly for consumer speaker installation. (High power commercial PA is a different animal.) If you can install Malibu Lights, you can install a landscape speaker. Conduit isn't required but you want to avoid putting a shovel through it while gardening. So some kind of protective cover or sleeving may be useful in some areas.

I'm a big proponent of individual wire runs and individual level controls for each speaker (with one master control for the whole system). There's added cost in the front end, more electronics, but I feel the added flexibility of setting sound levels for each zone is worth it. It's nice to be able to dial up the volume close to the pool where music is competing with pumps and waterfalls and swimmers splashing and have it lower on a patio where people are conversing.

Get a sonos amp and never look back. I run one for four out door speakers and two rock speakers. Mine are setup with a two channel knob type volume control on my exterior wall. All hardwired amd sound amazing. Sonos lets you stream from a multitude of music services all controlled via your phone or tablet. They are awesome. https://www.sonos.com/en-us/shop/amp.html