Yellow leaves on the bottom of the plant is the most common sign of nitrogen deficiency.

This can be caused by lack of nitrogn availability or other things limiting the uptake of nitrogen into the plant.

Overwatering: too much water can create anaerobic conditions (No oxygen). Bacteria in the soil convert ammonia and nitrate to N2 gas.

Any disease effecting the root. This includes fungal and bacterial root rots. Letting the pot dry outcan help reduce the pressure.

Other species that feed on the roots. Fungus gnats, nematodes, and many more things can feed on the roots.

I am the least-qualified person to reply but what I've learnt is that it's probably too much water, or not enough, or the correct amount but at the wrong depth. Or maybe it's not enough plant food, or too much, or not enough light (unless it's had too much but I think I can at least rule that one out). Or perhaps there's some pest or disease, or the plant's star sign means it must be aligned/unaligned with the local ley lines. Finally, your plant may dislike your music, or lack thereof.

I hope someone else can help narrow it down further.

This can happen just because of lack of sunlight. This happens to mine every fall/winter when I bring it in from living on the patio. Did you recently move it to a place with less sunlight?

That being said, when was the last time you gave it fertilizer? And how often do you water it?

Assuming it's this Baby Rubber Plant, you probably have root rot. Best bet is to move it to a warmer spot and avoid watering it for a bit and hope it dries out and recovers itself.

It looks like this is a commercial plastic pot inside of a decorative pot; it's impossible to tell if it still has water and you probably overwatered. The best bet is to repot it (ignoring the advice above), cut out some of the rotten roots, and move to a new pot with a drainage hole and a watering tray. If you want to replace the soil, get some perlite/pumice (those white rocks) for the potting mix. 1 part to 1-3 part potting soil should be fine. This stuff keeps air in the soil, keeping it dryer while also lowering soil compaction (a separate issue you may find in ~2-5 years).

Repot this plant if you're looking for a learning experience, otherwise buy a fresh plant with a draining pot / repot from the get go. I have not repotted for root rot yet, I usually opt to get a new plant and set up drainage in the first dormant season (fall/winter).

Looks like it’s too wet, and/or not sunny enough.