Here is a very old, but decent recording of a low tone Sterling M10 (dual head) siren. Do realize, that the M5 was intended as a fire siren, and lacks the characteristic "air raid" sound. I'm not sure if your version is single or dual headed, or high or low pitched (low pitch has 8 ports, high has 16 ports for the rotor/stator). However, I'm sure most of the people coming to your museum will not know the different sounds different sirens make. It's up to you whether you want to be more historically accurate (of which only one person out of 50,000 would know the difference), or make a more dramatic presentation, and use the sound clip of a true Civil Defense siren such as a Thunderbolt. Anyways, here is the recording. I'm not positive, but I believe Steelman Hiles made this recording around 1999/2000. He does not mind if this recording is used.
http://media.putfile.com/Sterling-M-10-low-tone-attack