Obviously these 2 pilots were on the proper frequency to receive their landing clearance initially, then either a transceiver failed, the #1 transceiver was changed to a incorrect freq. or the #1 transceiver volume was turned off.
If a transceiver was changed to a improper frequency most pilots would realize they might be on the wrong frequency with either departure/approach type of instructions or radio silence on a busy Tower Frequency. Then again these guys had another working transceiver that was most likely on a company frequency.
Another example of poor awareness of another AC crew.