Well, for pictures and sounds you would need to set up the app you are using to create them to save to the card. Or if installing from a computer, just choose the card (when plugged in via USB) rather than the device itself.
If you are downloading music from say Google Play, using Google Play Music, you can do the following.
1. Open the "Google Play Music" app on your device.
2. Go into "Settings", either by swiping into it from the side or tap the three lines on the top-left.
3. In the "Downloading" section tap on "Storage location".
4. Now select "External" from the menu. It will tell you how much free space there is on the card.