Most of the time, you hit a volume button on your iPhone, iPod touch, or iPad, and it does what you expect, whether it's turning up your reminder alarms or turning down your music. And then there are the other times.
But once you understand the different "kinds" of sounds, the interaction between software settings and the volume buttons, and how context overrides the default course of events, you'll have better control over your device's volume.
The sound of more than music
The key to mastering volume adjustment is understanding that most of the sounds on your device fall into one of two categories. General audio includes music and other media, and the voice volume on the iPhone and for FaceTime on all devices. The "ringers and alerts" category includes not only the iPhone ringer, but also: FaceTime rings; Clock app alarms; notifications and individual app alerts; keyboard clicks; and miscellaneous app sounds like the whoosh of sending Mail.
Basically, you can decide which kind of sounds--general audio or the ringers and alerts--your volume buttons control by going to Settings > Sounds and, under Ringers and Alerts, set Change With Buttons to On or Off; if you turn it off, the buttons control the general audio. But your default choice is easily, and often, overridden because what you're doing at any given moment takes precedence over the default settings
Context is everything
The volume buttons "just work" most of the time because they're context-sensitive.
If, for instance, you're in the Music app, the buttons change the media volume even if you've set them to control Ringers and Alerts; this happens even if the music controls are merely showing, on a Lock screen or in the multitasking bar, with no music playing. Conversely, when media volume is the default, you can change the Ringer volume when an alarm is playing. These changes affect the overall volume setting for that category, not just temporarily or for the current sound.
The volume icon that appears when you use the buttons helpfully indicates what you're adjusting. With Ringers and Alerts as the default, press a button when you're on a Home screen with no music playing and the icon is labeled Ringer (on the iPhone) or Sound Effects (on other devices). If Ringer and Alerts is turned off, you get the unlabeled icon that stands for general audio. If you're using headphones, it's their volume that's altered, not the device's speaker, and the volume icon notes that.
As for controlling the volume in games, that's complicated. Many games are set to the Ringers and Alerts volume when you first run them; some interact with that setting if you change the game volume. Some newly launched games inherit the volume level from the last game app you used. A game remembers its own volume setting from its previous use if it's been sitting in the multitasking bar. Luckily, no matter your default setting for the volume buttons, they will always control the volume of the current game.
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