if your mplayer is not in your PATH or is not named mplayer you have to set environment variable GAP_MPLAYER_PROG to your mplayer program and restart gimp
Zure mplayer zure BIDE-IZENean ez badago edo ez badu mplayer izena, GAP_MPLAYER_PROG ingurune-aldagaia ezarri behar duzu zure mplayer-aren arabera, eta gimp berrabiarazi