This concerns a suddenly loud car. When looking up at the muffler from below, a hole is visible. This is not a rusted muffler hole, but more like a small 1/4 inch mount for something? Should this muffler have some kind of plug for something that may have fallen out?
Just let a muffler shop look at it and it will repaired or replaced.
The hole is there to allow condensation to drain from the muffler so it doesn’t rust out.
Look for a larger exhaust leak.
When I have a suspected exhaust/muffler hole I’ll get a helper wearing sturdy gloves to hold an old rag or a board against the end of the exhaust pipe (loosely, don’t seal it off completely) while I’ll look under the car to see if I can see any exhaust smoke going out the hidden hole further upstream. It’s easier to do this when first starting the engine on a cold day b/c there’s a lot of visible steam in the exhaust gasses then.