Community Fusion tools
Fusion, Resolve’s compositing page, can be extended with “fuses”, small tools written by users. The best known for this is the community AudioWaveform fuse, installed through Reactor, the community package manager for Fusion. It draws a waveform from an audio file you point it at. It isn’t made or supported by Blackmagic Design, so check that it works with your version of Resolve before relying on it for a deadline.
Paid and free templates
Template sellers offer Resolve “audio visualizer” packs as Fusion titles or macros. Many are driven by keyframes or by a separate audio file, so read the description: a pack that “reacts to audio” should say how. Try one free template first to see how it behaves in your Resolve version.
Building it from scratch
Without a fuse, a reactive effect in Fusion means converting the audio’s loudness into keyframes by hand or with a script, then linking those values to the size of shapes. That is worth it for a signature look you will reuse, and too much work for a single release.