Both 聲 and 音 are in the auditory domain, which can denote the sound, thus, both of them can inherit common semantic information from the upper concept, namely sound.
we think that the similarities on synaesthesia between 聲 and 音 are determined by the whole synaesthetic characteristics for hearing, that is, hearing is a productive recipient of synaesthetic transfers and is much easier to be the target domain of vision than touch.