The Logic 7 amps fail quite often but will affect all low range speakers in the rear and under the front seats. If the optic receiver/driver is failing in the L7 amp it will affect the complete MOST bus.
The ASK will affect the high range speakers in front.
But first you need to do a light intensity test on the MOST bus to see which component is dropping out.
The easiest test is to loop out the most common failure items on the MOST bus one at a time such has.... DVD NAV, SVS module, and TCU. Reset the MOST bus and listen for presence of the crackling noise.
Use BMW 61 13 6 917 541 MOST fiber optic Loop.
The 4 modules listed including the L7 amp are in the trunk behind the left trim panel. The L7 amp if used is plug and play no coding necessary.