Earpiece speaker not working or very faint even after replacing it

Hi,

I started my own iPhone and Mac repair business at home when Covid hit. Over the last 1.5 years I gathered quite a substantial amount of knowledge on how to repair iPhones and MacBooks. So  suddenly I am getting all these iPhones in with earpiece speaker problems. I literally can't hear anything during calls when I put it at my ear and it's very faint when I enable 'Hearing Aid Compatibility' in Settings. Over the last week I replaced the earpiece speaker for an iPhone X and iPhone 11 Pro by soldering it onto the original ribbon cable. I also replaced the whole front ribbon cable for an iPhone 8 Plus and replaced the earpiece speaker as well. In addition to that I cleaned the earpiece speaker grill with methylated spirit and a brush. It doesn't solve the problem for any of them and I still have the same problem. This let me to believe that it could be a software related issue since this is happening for various models. The iPhone 8 Plus is running iOS 14.4, the iPhone X iOS 15 and the iPhone Pro iOS 14.8.

Thank you everyone for your help.