As mentioned, the site is focused on animation, and adding sound would probably cause more copyright issues. I don't think storage was a factor (480p videos on the booru can reach 5,000 kb/s versus ~128 kb/s for the average streaming audio track).

Also, I'm not sure if this is something that was taken into account when the site was created (it was probably just these 2 reasons above), but a lot of posts on the site are edited so that the animation flows better (even if the site tends to move away from this, it can be useful when the cuts of an artist are sandwiched between other cuts), so it would give very odd results on these posts.