Even though the sounds of Disaster Relief can be instrumental in the sense that they do not have any vocals or singing, the music seems to be able to sing the song to the point where a voice added in might feel like too much. The way that these songs follow a somewhat traditional rock base with the guitar, bass and drums but then add in other instruments (a horn section mainly) give them a whole new vibe.
With the horns and the way that the guitar riff sounds sometimes Disaster Relief can definitely feel like ska. In terms of existing bands, this can be compared with an artist such as Catch 22 and thus also Streetlight Manifesto, who are both different types of not quite ska but more of a rock n roll based ska. When it comes to bands that I've heard which sound similar to Disaster Relief perhaps only Sweep The Leg Johnny comes close.
What becomes difficult to describe in this music is the way in which it is all blended together and can change genres at any given time but with such deliberate purpose. When you think of ska and how it is essentially defined by the horns, you also have to think that those horns- the sax for example- can bring about the sound of jazz and so at times this does feel like it has elements of jazz, such as on "Beach Song".
The overall sound can still feel like rock n roll, which can be demonstrated by this just stellar guitar solo during "Weekend Čoček", but those elements of rock can sort of blend in to become funk (like the way "Lowrider" and classic rock can go sometimes) and also just blues. So even though this album might seem like one sound on the surface, all of the sounds which feel related to that can make their presence felt and for that you should be listening to this album closely.