It may that that thermal acoustic power is enough.
If 1% of the lasers 5 mW is converted to absorbed acoustic power, we get 50 uW, which would amount to about 77 dB effective SPL at the microphone, which is fairly loud and would be easily picked up by a cheap microphone.
If 0.01% is converted, you are at 57 dB SPL, which is quiet, but still audible.
Feel free to poke holes at these numbers. It's been a long time since I studied anything acoustic.
Cheers,
John