Please don't hold down Sleep/Wake and Home. That yanks the power without a proper shutdown. You can corrupt things/lose data. Don't do it! Just hold down the Sleep/Wake switch on top, then slide to power off. This will shut the phone down the proper way.
A restart will temporarily correct the alert volume issue, but only until you use Siri or turn-by-turn directions with voice. Then, it will trigger the bug again.
You can just go into Music and turn up the volume, which will also raise the volume of the alerts, and you don't have to restart.
This is an annoying bug. Apple needs to fix it with an iOS update.