Sonobuoy
The Sonobuoys that will be dropped as listening devices in the larger search area are indeed broad spectrum passive listening devices. The modifications to the aircraft equipment might allow for a narrow frequency spectrum to be more closely monitored so that the returned signal will only scan the chosen frequency range around 37.5kHz.
Depending on the pattern of the sonobuoys and the contact gained on one or many of the buoys, he contact could serve to localise and triangulate a more refined position of the signal source. This technique has been in use for decades but probably not used to date because of the modifications required. The buoys can be monitored by one or several aircraft at altitude depending on the number of buoys deployed and the number of receiver channels on each aircraft.
The limited depth of the sonobuoy hydrophone (about 300m) could make detection through varying thermal layers less likely with weak signals. At least all resources are being employed to solve this mystery.