It depends which version it is.
The PA15As that were made in the late 1960s have serial numbers beginning with 1B, 1C, and 1D and have low-pitched wail and yelp tones. They are definitely collectible and are period correct for any late 1960s to early 1970s emergency vehicle restoration.
The PA15As that were made in the 1970s have serial numbers beginning with 1E and have high-pitched wail and yelp tones that sound similar to the electronic sirens that are in use nowadays. There were a lot of series 1Es made and they are easy to find. 1Es are collectible, but in my opinion they are not worth as much as the earlier versions.