Mix Universe

It takes a minute to get to grips with the game, but once you do, it's really cool.
There are two in-game tutorials which will get you going on basics, but really to do anything interesting you need more than that. I strongly recommend watching some of the videos linked on the main menu, at least the first two.
I've only just started playing, really, but now that I understand the core concepts, I'm sitting here with a dumb grin on my face at getting an automatic piano base going. Is that music in itself super special? Definitely not, but I'm really excited to keep going and see how far I can push it, and the artistic vibe of the world you're doing this in makes the entire thing super relaxing and rewarding. You're not just playing some basic piano track, you've got this entire visual experience punching along with your beat. It feels great.
The game works with MIDI keyboards as well, for those that are wondering. I'm playing without one myself, and the UX is more than good enough to get things going without, but if you do have one kicking around it's probably worth getting it set up for this.
If you're at all into music synthesis - or have contemplated whether music synthesis might be something for you but haven't bothered due to the complexity (that's me!), I really recommend giving Mix Universe a shot!

Even with no music theory knowledge, I managed to create something that sounds pretty good, once you get used to the nodes and how they behave/react you can create some pretty cool music, this is the sort of music creation game/tool I was looking for, very intuitive and easy to get started

Disclaimer: I've racked up 150+ hours in Mix Universe between early access and playtest builds—much of that time was spent creating starter content that was included with the early access launch, which is why I received a copy of the game.
Mix Universe takes the term 'music sandbox' to heart. It's clear that a lot of work has gone into making nodes that are accessible for folks who are new to making music, while still having plenty of depth for the seasoned producers out there. The addition of terrain-editing was a really cool surprise—it brought out a whole other creative side of me that I didn't know I had.
The UI can seem a bit overwhelming at first, but it's likely to become more streamlined over time. There have already been some really huge QOL improvements and new features since the playtest, so I can only see that continuing to happen throughout early access.
Take it for a spin if you're at all interested in making music!

Mix Universe is on the right track. It's fun and easy to loose track of time with.
You don't have to be musical to understand it.
It's the coolest creative music world builder game I've ever played.
Plenty of great examples projects and tutorials in-game to learn from too.
Looking forward to the future with this!

Amazing game! So relaxing and visually stunning. Wish there was a simpler "entry mode" where I could just walk around other peoples creations but the mix samples is essentially that. Can't wait to see where the dev takes it.

TLDR: A fun way to play with sound and craft musical expressions within galactic themed landscapes and scenes.
For transparency, I've had the pleasure of helping test this game in it's early stages which is why I received a copy of the game. I've seen it evolve and improve through the efforts of a single developer with input and feedback from a small community of early playtesters. That being said...
It is exactly as advertised, a "musical sandbox." Like how in city builders you create a city by constructing buildings then connecting utilities and roads between them. Or like how in a crafting game you build things block by block or component by component linking them together to form something bigger. In Mix Universe you are building musical expressions by creating, organizing, and connecting individual sounds in different ways.
It is not a digital audio workstation (DAW). So, don't expect all the capabilities of a DAW, but it has many similar features. You can use a midi keyboard to set notes if you like, apply different FX to the sampled instruments to sculpt your own sounds, and certain nodes can be used like a sequencer.
There's also the aspect of being able to place some scenery objects like asteroids and landscape as well as visualizing elements that are reactive to the sounds in the environment. Being able to create the landscape and scene that holds one's music adds another layer of depth to the experience.
This is friendly to non-musicians. It has a couple of tutorials available so that one can understand the controls and how things are connected. Hopefully there will be more tutorials be available in the future. As a musician, I've found it has helped me better understand timing and subdivision by having to think about music notes and phrases from a modular perspective.
One can get the "fear of a blank canvas" when first launching the game, but don't worry it's a whole lot of fun once you get your bearings. For me, it's a great creative outlet.