We show by computer simulations that acoustic emission from the crack tip strongly reduces the delivery of fracture work, due to the coupling between the crack speed and the acoustic branches in dispersive media. The direct consequence is a selection criterion for the terminal crack speed which, for planar cracks, produces results corresponding to those found in experiments on highly anisotropic materials. In case of isotropic material with cracks of unrestricted geometry, the drop in the crack speed with respect to the planar case is connected to a mechanism of attempted branching, which is also responsible for the logarithmic roughness of the final fracture for marginal loadings. Higher loadings lead to a well defined roughness exponent of ζ ~ 0.45 compatible with that measured experimentally at short length scales, and in our simulations clearly connected with the generation of macroscopic branches.