how to split the raw music into pieces aligned with motions

[ZeyuLing]

Hey, i'm wondering how to get the music piece exactly for a motion.For example, the motion is gBR_sBM_cAll_d04_mBR0_ch01, it only covers part of the beat mBR0. How can I cut the music piece to get the part aligned with gBR_sBM_cAll_d04_mBR0_ch01 exactly?