Some clarification on this: the sniffer seed farm currently works by having allays gather up the seeds and deliver them to a collection point (hopper minecart rail underneath was also an option but allays seemed simpler). In order to do this they need to be holding one copy of the item they are assigned to gather. If you take that item away from them they will not collect the dug up seeds anymore.

The note block triggering periodically keeps them from flying away and keeps them delivering what they pick up to the right place so it can be collected via hopper. The farm was purposely built on the outskirts of town so the noise would not bother people. If you find yourself having to spend a lot of time near the farm and find the noise annoying you can turn down the volume of note blocks in your game options.

Above the note block there is also a jukebox that you can use to duplicate the allays in case the collection capacity needs to be expanded, e.g. if you see seeds lying on the ground and not being picked up in time because there are too few allays (or if allays inexplicably go missing as has happened once). You can also feel free to duplicate allays if you need some for yourself, but leave the ones holding the seeds in the farm.

Addendum: the sniffers and allays in the graveyard inside the city itself are mainly there for atmosphere purposes - hence the joke names - they are not supposed to be a second farm, though the allays may occasionally pick up some seeds. If you want them to drop what they have collected ring the nearby note block once by left clicking on it.