Finishing raw MDF cabinets - oil, paint, or veneer?

Paint is what I've landed on for my last four builds and I'd do it again. Your rounded baffles actually push you that way, since veneer on curves is a real headache and iron-on edges on a baffle corner tend to lift if you look at them wrong. Oil looks lovely but I had a pair in wipe-on that picked up a dent from a vacuum hose and the mark never relly healed because the finish stayed a bit soft, like you suspected.

On the does-it-matter-for-sound question: for a sealed or ported box it's basically all about looks and scuffs. The only way a finish affects anything is if the cabinet leaks air somewhere and you happen to seal a gap while coating it, and honestly a bead of caulk on the seams inside matters way more than whatever you put on the outside.

Since you've got trim paint already, roll it on thin. I learned the hard way that heavy coats on MDF raise the fibers and you get fuzz. I do one thin coat, let it dry, hit it with 220 lightly, then two more coats with a light scuff between each. No sanding sealer needed if the paint's decent. The MDF edges will drink the first coat, so expect the cut sides to look rougher than the faces until coat two or three goes on. A cheap foam roller from the hardware store leaves a nicer texture than the fuzzy ones. Total time on my last pair was maybe three evenings and nobody who visits guesses I rolled them.

I'm in the veneer camp and honestly the rounded baffle worry is smaller than it feels. My first pair had quarter inch roundovers on the front and I used a heat-lock veneer with a household iron, working from the center out and clamping a flat block down as each section cooled. The corners took maybe ten extra minutes of patience and two years later nothing has lifted. There are also flexible paper-backed sheets that bend around gentle curves without fuss.

On the sound question, no, the finish is not doing anything audible on a box that thick. A coat of paint versus oil versus veneer is purely looks and durability, so pick whatever you'll enjoy staring at.

That said, your leftover trim paint is not nothing. I rolled trim paint on a set of surrounds once and it looked fine at a distance but every roller stipple showed under window light. Sanding lightly between coats with 220 and thinning the second coat a touch is what saved it, so if you go that route, plan on three coats minimum.

One middle option nobody ever seems to try: oil the boxes and just accept they're furniture that will pick up character. Mine have dents and my kids' sticker residue and I've stopped caring. The cabinets sound identical to the day I finished them.

I am leaning towards the paint because the idea of ruining a whole box with some sort of veneer is too scary for my first project. I had a few big bottles of oil stain sitting around and it seemed like the smell would just kill me while I worked, and I worry about getting covered in it. Does anyone know if using a small room ventilation fan really helps with the drying time when painting those big flat surfaces?

I'd go with a primer on the bare MDF before any paint, expecially on the cut edges. Those raw wood surfaces will soak up the paint like crazy and it can lead to uneven color or issues later if you try to touch it up. I always grab a can of shellac-based primer for this kind of project. It seals the fibers first and stops the paint from looking blotchy. Even if you are just rolling it on thin, that extra layer of base material makes the job much cleaner. If you go with paint, just make sure the primer is fully dry before you start with your first coat. I found that cutting down on drying time with a small fan helps a lot with the fumes too.

I'm leaning towards painting because I've never seen a veneer installation go wrong, wich sounds like a lot of effort. My main worry though is how long those trim paints take to fully cure, especially since my workshop doesn't have great airflow. I'm wondering if doing a super thin coat first just to seal the MDF surface is actually a thing, or if that just complicates the whole process.

Update: so the primer tip saved me. Sealed the bare edges first like was suggested and the paint finally went on even instead of soaking in blotchy. Still wrestling with the baffle though, the rounded corners show every roller mark and the last two coats left a slightly lumpy feel. Did anyone here thin thier trim paint a bit for those curves, or just roll it light and accept the texture?