Use a DC-to-DC voltage booster to convert battery voltage into higher voltage enabling you to drive an external audio amplifier. This will give you a more solid sounding unit.
Because output ripple is much lower after DC-to-DC conversion, this can also reduce any motor "hum" sound during motor start up and at low speed. Another benefit of this configuration is that it will deliver a much louder volume with an 8-ohm speaker.
Click here for more information and connection diagrams (bottom of page - opens in new window)