Can "somethings" be used as a plural?

Well, something is not a noun to be pluralized. So you cannot say somethings. However, there's one thing about something that can make it a noun, and that's if you prefix it with a number. Here's an example:

she writes a column geared to twenty- and thirty-somethings.

Technically it is possible to write

I heard the soft thumps of somethings heavy on cloth.

something

n.
3. a person or thing of some consequence.

cf. nothings

n. 1. inconsequential conversation; as, they traded a few nothings as they parted.

It is not employed in this sense very often and sounds odd, though.

The author's idea is obviously to suggest that more than one 'something' was involved, the plural thumps clearly shows several objects as the cause of the sounds. The tricky part here is that all these objects together constitute the focus as one, just as the 'thumps'.

There's neither a grammatical nor a semantic error in the sentence. On top of that, the author has 'literary license' to play a bit around, too.

Gerard Ellis (Ed.), Conceptual Structures 3, 1995, p.310

A mind is able to distinguish somethings, a word we use for perceptions and other awarenesses. We suppose one, outer, world and many sets of somethings, as many as there are …

[Emphasis added]:
Sextus (Empiricus), Against the Grammarians (Adversus Mathematicos I), 1998, p.6

… if something is taught, it will have been taught either through not-somethings or through somethings. But it is impossible for it to be taught through not-somethings, since these are not subsistent for the mind, according to the Stoics …

Swartz, Ballet et al., Saint Augustine, Kent et al., Hartshorne, …

It can if you have a valid and up-to-date poetic licence.

Reminds me of The Cat in the Hat Comes Back by Dr Seuss, and his pluralization of "somebody":

When our mother went down to the town for the day,

She said, "Somebody has to clean all this away.

Somebody, Somebody has to, you see."

Then she picked out two somebodies: Sally and me.