First you must have this information:
1) Does the speaker work with AC current or DC? If it works on AC you'll need to convert the DC output of a battery to AC using an inverter.
2) And how much current does it require at it's maximum output, for e.g. 65W?
With these two numbers you can choose a battery, and eventually the other pieces you need to operate the speaker. For a battery you can calculate how many watts/hours it can give you, for e.g. a 65Wh (Wh = Watts per hour) battery would make your speaker work for 1 hour (in a DC current scenario, in AC you'd have a % of loss given by the conversion process from AC to DC).