I noticed that also. One set of shots very loud and another set very faint in comparison. At that moment I thought there are two shooters.
Faintness could be caused by shifting wind or a group of people passing by or the shooter stepping back from or closer to the window.
Also, the person holding the phone could have moved their hand to a different position.
Example - When sitting at my desk I have to turn the tv volume up to hear it because the couch is blocking the sound between me and the tv. However, it’s very loud when I stand up. Just that bit of movement makes a world of difference. There’s also a difference when the recliner goes up and down.