There is no consensus about selah. One opinion is that it means take a solo. Another, to syncopate the rhythm. Yet another opinion is that it means take a break, musically. Then there is the possibility that it means ornamentation. The thing is, nobody knows what role the music and the instruments played. Some psalms have instrumental backing, others are a-capella. I doubt that their concept of accompaniment was anything like what we've gotten used to since the 1960s.
I think the closest we can get to understanding the music of the psalms, is to examine how the Yemenite Jews recite them. It's not quite singing, more like long intonations, but very interesting and quite a feat.