So, sometimes when I'm at work, I have a problem wherein I want to use my phone's data connection to stream music, but I don't want to be bombarded with notifications from Google Voice/ GMail / Facebook / etc. Is there a way to disable the data connection for those applications, so that the notifications don't pull me "out of the zone", while still leaving the phone's overall data connection intact so that I can stream music?
You can turn sync on/off for each app individually from the main device settings. Under the accounts list, click on an account to turn sync on or off for that account.
Alternatively, you can simply turn off notification sounds for all apps. Use the volume rocker to bring up the volume control, then click the settings icon to expand the volume control into separate controls. The speech bubble with an exclamation mark is the volume for notifications: drag that slider all the way to the left to mute notification sounds without affecting music volume. There's slider with a phone icon lets you mute the phone ringer independently.
In addition, some phones have a blocking mode or driving mode which disables notifications completely. On recent Samsung phones, you can reach this from the quick settings panel in the notification tray. Tapping the icon in the quick settings panel turns it on and off, and long-tapping it takes you to the settings for that feature.