volume is not always consistent

Hi adriann09_,

The first steps to review for a concern of this nature are outlined in the article If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker.

Specifically note the additional steps you can try from that page of:

"If you want to try more steps on your own

If you prefer, here are some more steps you can try on your own. After following these steps, you might still need to contact Apple Support to discuss service options.

1. Remove all screen protectors, films, or cases from your device. 
2. Check the speaker opening to see if it's blocked or dirty. On iPhone only, make sure that the receiver isn't blocked or dirty. 
3. If necessary, clean the speaker or receiver opening with a small, soft-bristled brush. Make sure that the brush is clean and dry.
4. Go to Settings > Sounds (or Settings > Sounds & Haptics), and drag the Ringer and Alerts slider back and forth a few times. If you don't hear any sound, or if your speaker button on the Ringer and Alerts slider is dimmed, your speaker might need service.
5. If you hear sound, try making a call with your iPhone and turn on speakerphone. On iPad or iPod touch, make a FaceTime call. If you still can't hear, or hear static or crackling, then your network or reception could be the issue. Try to call again later, or from a different location."

You may also check for active Focus states, nearby Bluetooth devices that might be pairing, and if you use an Apple Watch this may be expected as notifications will go to that device if your iPhone is locked.

Regards.