You won't be able to play audio while the board is connected to USB, so to actually trigger audio you'll need to unplug the USB cable and power it another way. There's many ways to power the Sound Board, you'll have to go with whatever it easiest for you.
This is by far the simplest way to power and doesn't even require any soldering! Simply connect the board micro USB to a USB power pack. Since there's no data lines on a battery pack, it wont try to start up as a USB key. Just connect with any microUSB cable. These are large but it will power the Sound board + amplifier for a looooong time
The down-side of using these is that some power packs automatically turn off when they don't detect a lot of current draw immediately. Its impossible to tell ahead of time whether your power pack will work without testing, but if your project doesn't make music sounds almost immediately, it can be an issue
The 2x2W amplifier version of the Sound Board is a pretty beefy 2x2W amplifier. If you are using 2 x 4 ohm speakers, that means you could be seeing as much as 2 Amp spikes of current. (In reality, assume spikes of 1A since audio isn't a DC load). Power with good charged batteries like AA's or even AAA's. For lipoly batteries, a 500mAh battery is minimal! Go for 1200mAh or more if you plan on playing it loud
You can use a AA or AAA battery pack and wire it up directly to the Vin an GND pins. We recommend 3xAA or 3xAAA alkaline or rechargeable as ideal. You can use 4xAA or 4xAAA but in that case, make sure you've got rechargeable batteries, as the higher voltage alkalines may be a bit much for the board.
If you don't want to solder wires directly, you can solder on a JST PH 2-pin connector to the back. We don't include this on the board because its a bit bulky and if the JST is in place you can snap a battery in or out easily. The connector is the same as the Vin/GND pins on the side so use one or the other!
We designed a Lipoly backpack for the Pro Trinket but you can also use it with the Sound Board! Use the extra long header included to solder it over the USB jack, it connects to Vin, GND and BUS
The Lipoly backpack lets you plug in a lipoly battery of any size and recharge it over the MicroUSB connector, so it's basically an all-in-one portable + rechargable system.