Which is better here, colon or dash?

Yes, you can (though it's probably better with the dash, as was already mentioned in the comments).

Discussion

Let us simplify your original sentence to

[A] There is no beat, melody, or rhythm – only techno-cacophony.

The part after the dash, only techno-cacophony, is a supplement. Supplements are parts of a sentence that aren't syntactically integrated to the rest of the sentence. Instead, they are only semantically related to another part of the sentence, the so-called anchor. (For an extended discussion of supplements and anchors, see e.g. this answer.)

On the face of it, the supplement in [A] is a noun phrase (NP), whose head is the noun techno-cacophony, and the adverb only is a peripheral modifier (CGEL, p. 436). However, from the semantics of [A] it is evident that the the anchor is the whole initial main clause, There is no beat, melody, or rhythm. But according to CGEL, supplements in the form of NPs have other NPs as anchors (pp. 1356-1358). Therefore, it seems reasonable to postulate that only techno-cacophony is in fact an ellipsis, and that the full, non-ellipted sentence reads

[B] There is no beat, melody, or rhythm – there is only techno-cacophony.

The part after the dash in [B], there is only techno-cacophony, is now a supplement in the form of a main clause, and such supplements indeed regularly have other main clauses as anchors (CGEL, p. 1359). Here is what CGEL says about such cases:

Supplement main clauses in final position (especially those without any indicator [e.g. namely, that is, etc.]) are not clearly syntactically distinguishable from separate sentences. In speech, one can use intonation to link a clause to what precedes as supplement to anchor, and in writing punctuation serves to mark more explicitly whether a main clause is being presented as a supplement to what precedes or as a separate orthographic sentence.

Supplements of this type are commonly preceded by either a dash or a colon. CGEL gives the following example that uses a colon:

I raised a more serious objection: it's against the law.

Here are some examples of similar constructions:

Dialogue between the two presidents remains essential: only that can create the political space for civilian and military officials in both nations to engage with one another in discussions that could prevent catastrophe.

^{From Ernest J. Moniz and Sam Nunn, 'The Return of Doomsday: The New Nuclear Arms Race-and How Washington and Moscow Can Stop It', Foreign Affairs, September/October 2019}

We don't publish any letters: we only accept commissioned articles.

^{From CGEL, p. 383}