Are the beams above or below the notes?
R, 40 bytes



function(x)min(x)+max(x)+mean(sign(x))&lt;0


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If min(x)+max(x) is non-zero, its sign corresponds to the note farthest from the centre and gives the direction of the beam. 

Adding mean(sign(x)) can never change the sign. Indeed, either (a) all notes are of the same sign and mean(sign(x)) is of the same sign as min(x)+max(x) or (b) notes are of both signs and mean(sign(x)) is strictly in \$(-1,1)\$  whereas min(x)+max(x) is an integer, so the sign of the sum is unchanged.

If min(x)+max(x)==0, the sign of the mean decides the direction. 

If undecided, defaults to FALSE.