All the above are good workarounds. Headphones can be used inventively: Gertrude Art Spaces here in Melbourne, Australia had a show of 100 sound art works recently in a space about the size of a small house, pretty much by hanging 100 sets of headphones from the ceiling.
The ideal solution, though, would be for future galleries to be designed with acoustics in mind too.
Traditionally sound-based works and visual art occupy different spaces and regulate movement differently. Places designed with sound in mind - theatres, concert halls, cinemas - assume a seated audience with their attention in one spot. If there are multiple works these are ordered in time, not space, and the audience encounters them in order. Architecturally they're designed to broadcast a single work throughout a large space.
Most art galleries, on the other hand, assume a mobile audience and multiple works ordered in space, not time. (It'd be entertaining to set up a gallery on the broadcast model, where the audience walks in, sits down, and stares at, say, a painting without being able to wander around. Heh.)
It gets tricky when artists start wanting to use a gallery setting to present works where the sound is important, especially when there's more than one work with sound at a time. But then I think about my local multi-screen arthouse cinema, which manages to have about 8 rooms, none of them all that big, which are sonically isolated from each other and sound decent. It makes me think of a space something like a tiny multi-screen cinema with 80% of the seats removed.
The ideal space would have non-parallel walls, a mix of absorbent and reflective surfaces and, I guess, doors between different exhibits. Plus relatively soundproof walls. It'd cost a lot more than an ordinary art gallery to build but probably a lot less than a concert hall. I daydream about setting a place like this up one day! How it would fund itself is a tricky question: sound-based works seem harder to sell than paintings and sculptures, and government funding can be erratic. Dreams are cheap though..