Why does my iPhone 4s only vibrate for texts when phone is charging?

My iPhone 4s will ring just fine for calls and texts unless I put the phone on the charger. While charging it will ring normally if someone calls but if someone texts while its on the charger the phone will only vibrate. I've checked the settings, do not disturb is not on, the button on the side is not set to vibrate (no red showing), I've done a hard reset, I haven't done an update so I doubt it's a software issue, I've tried cleaning my iPhone to remove any possible lint, and I have my texts set to ring with a ringtone but still no luck. It's been this way for the last 2 weeks now and I'm wondering if anyone has an idea of how to fix this!

My iPhone 4s will ring just fine for calls and texts unless I put the phone on the charger. While charging it will ring normally if someone calls but if someone texts while its on the charger the phone will only vibrate. I've checked the settings, do not disturb is not on, the button on the side is not set to vibrate (no red showing), I've done a hard reset, I haven't done an update so I doubt it's a software issue, I've tried cleaning my iPhone to remove any possible lint, and I have my texts set to ring with a ringtone but still no luck. It's been this way for the last 2 weeks now and I'm wondering if anyone has an idea of how to fix this!