What exactly should I do? Download source file and extract it to the folder?
I'm not familiar with Kubuntu but in case you want to try updating SDL libraries (or audio drivers) there must be update manager within Kubuntu, though I don't know what it's called.
But in case of trying manual audio hack, you can find all the game samples in folder called audio/ in your game's installation folder.
There you can find rain.ogg which is a sample for rain. Now if you have an audio editor (audacity for example) you could try to open rain.ogg and save it back with different parameters (if some are available). Or maybe just open it, crop it a bit, and save back with audio editors .ogg defaults.
There's no guarantee this will fix things, but the issue sounds like audio decoding problem.