This is caused by the pressure release in the brake lines when you retract the pistons in the brake calipers to accommodate the new brake pads that are thicker than the ones that are removed.
You need to inspect the brake pads/shoes and have the brake system bled. The soft pedal is air in the brake system. The fact that you are hearing air makes me suspect you may have a defective brake booster. Have the power brake booster vacuum diaphragm inspected.
Air in the brake lines
Brake pads could be worn or lack of fluid in brake funnel
brek pads
You need to bleed the brakes. Loosen the bleeder valve on one of the calipers and have someone press and hold the brake pedal. Tighten the valve and release the brake pedal. (If you let off of the pedal with the valve loose, air will be drawn into the system and make the problem worse.)
Brake pads are designed to give us a warning before it is absolutly necessary we change them, you will notice a soft squeal, (this is by design) as the brake pads wear the squeal will become more noticeable. Once you start hearing a squealing sound, start planning trip to the shop.
There is still air in the system.
Air in the system. Bleed the brake system.
Soft pedal is caused by air in the system. Bleed the brakes starting at RR, then LR, RF, and ending at LF.
Wornb brake pads. You are hearing the warning device that is telling you to replace the pads.
The brake pads only cost $12 and they are very easy to install, so the minute you hear a strange noise when you apply the brakes, you should change them to prevent damage to your rotors. You can also check the wear on your break pads to see how much padding is left. If they look thin, then change them. There are more signs to a brake problem. If you feel your brake pedal kinda go up and down (pulsating) your rotors need to be replaced. Soft or spongy brake pedal again could be brake pads to thin or air in the brakes, etc. While braking your car is being pulled to the left or right this is a sign of one or more calipers starting to seize.
if brake pedal is soft , either there is air in the system ,a brake fluid leak, or brake linings and pads are worn allowing the pedal to travel to far if there is a fluid leak, this can usually be found by getting under vehicle and looking at inside of wheels for brake fluid