There is some disagreement as to whether you must inform in order to audio record when in a public location. Several circuit courts (but not our 9th) have gone on record with precedent that it is protected activity so long as it is not done clandestinely (i.e. audio recorder, cell phone, or video camera in hand and not hidden) while in a public place.
However, since the 9th hasn't set precedent for us here in Oregon, the other circuits are useable but not precedent that must be followed and you could end up a test case. Best bet is to either turn it on after you say "I am audio recording" or leave it on and wear say that to everyone you come in contact with....not particularly endearing to folks you don't know. You could probably get away with a sign that says "I am audio recording" but there is no precedent in the 9th for that either.
The one thing that you should be well aware of is that you DO NOT need PERMISSION to record, you just have to inform that you are (to avoid any possibility of becoming a test case).
