There should have been a Giant Toad appearing as they enter the swamp telling them to stop their singing...
Combat against an unknown force is difficult. Do you group up and by chance get swarmed, or do you spread out to a point where you can see one another without fear of hitting allies. Neither being the better choice.
ATM, due to it being fog, Melee is the best course of action. As a Range weapon needs time to work out the aim needed and when you can't see something till the last moment bows and arrows would be shit. 2/3rds of the Bowelves Swap out bows for Swords/Spears Group together at a reasonable distance, move the caravan towards the center. Use the spearelves to cover the rear by forming a phallax, while the unarmed elves who are between the rear guard and the caravan collect random stuff on the ground to pelt at anything that attacks from their direction... Those with Swords are the front guard. Spread out at arms distance so that they have enough space to swing while 1/3 of the Bowelves stay equipped with bow and use the space given to set up quick volleys when available. Not wildly firing. If anything if the Swordelves are face with a large swarm from on direction, they should crouch, the bows fire, then the swords counter the remaining that was not hit by arrows...
All this if the 'voices' are hostile and killable... ^_^