I don't have the guide myself, so I'll have to refer you to the article on the Age of Legends at the Thirteenth Depository. It's under What Were Da’shain Able To Do Because They Follow The Way Of The Leaf?" and says that the Da'shain could enhance Channeling with their Singing. One can also extrapolate that they could do other things as well, for example in The Shadow Rising, The Dedicated, Da'shain Aiel sing to the mad male Aes Sedai Jaric Mondoran to try to calm him down. It doesn't work, or at least, Jaric ends up killing them hours later, but we can surmise some curative purposes to some songs.

Also, there is the Voice that Lews Therin mentions in Dragonmount, which also involves Singing, but can't be the same as the Da'shain, since none of the Da'shain Aiel can channel, and no channeler is a Da'shain Aiel...

'Course, aside from that Channeling-enhancing Singing, we can't really know what else it could do, but we can pretty confidently determine that it could do other things than crop growing.

EDIT: And yes, those songs have been lost... but then, so has seed-singing, so I don't see what difference it makes .