Can´t adjust volume when proximity sensor is active

Hello noeaguirre,

The first thing we would suggest would be to power cycle the iPhone by following these steps found at Restart your iPhone:  

1. Press and hold either volume button and the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, Force restart iPhone - Apple Support

3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.

While simple, this can refresh the iOS, apps and connections. 

Since it seems the volume from the speakers is low, you may also want to look through the information outlined here that has steps to help resolve that issue: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker

Have a good day.

Hey noeaguirre,

Does the same issue occur when using the Phone app on your iPhone? If you haven't tested that yet, you may want to as it could provide a little more isolation. For example, if the issue doesn't occur when using the Phone app but does occur in other apps, those apps may need to be updated. Learn How to manually update apps on your Apple device here. It's recommended to attempt an update of apps periodically, even if you have it set to automatically update. Give that a try and see if you notice a different behavior.

Take care.

Thanks so much for verifying that, noeaguirre,

Since you've exhausted troubleshooting, and this is still occurring, we'd advise to contact Apple Support, so they can look further into what's happening.

Apple Support

Let them know exactly what's happening, when you noticed it starting and the troubleshooting you've done.

Make sure to mention it's happening with the Apple Phone app also.

Cheers!