The passive array equipped buoys are fully capable of picking up the DFDR/CVR pingers.
Buoys might have limited value in this search.
The pingers can nominally be heard for about 2 miles (10,000 ft) The depth of the ocean in this area appears to be about 10,000 feet, but varies between about 8,000 feet and 12,000 feet. It would be pure luck to drop a monitoring buoy in a position where it heard a pinger, given the depth in the area, and the fact that at the surface, the radius of success is substantially smaller than 2 miles..
The distance the pinger can be heard will also be affected by any thermal layers in the ocean, and the terrain where the wreckage came to rest, both reducing the range..
The search won't begin to be effective until towed sensors that work below the surface are deployed. I suspect they are being flown in as we discuss this.