Should I Use a Diode to Protect LiPo Battery?
Yes, you can use a diode for added protection.  Go with a Schottky diode that has a low forward voltage (to minimize power loss) and ensure that it is capable of handling the power (I*Vf) that you require.  You should also be sure that you have in place a way of stopping the battery from becoming over-discharged.  This irreparably changes the internals of the battery.  This is not usually a problem when it is discharging, but past a certain point (~3.0V/cell), it becomes increasingly hazardous to continue charging and using it.

It's worth mentioning that if your setup looks like this







simulate this circuit &ndash; Schematic created using CircuitLab

then A) the DC/DC converter will always be using some amount of power and B) installing a diode is as much about protecting the converter as the battery.  If you install a diode, you would do so after the converter but before the switch.  If there is a voltage feedback pin on the converter, make sure it is connected after the diode, otherwise the speaker will receive 12V - the voltage drop across the diode.

Lastly, as Jason says, there are other battery chemistries that are better suited for low power loads (such as your speaker, most likely).  If you really want to use a LiPo battery though, there are several with integrated battery protection (over-charge, over-current, over-discharge) which would take care of much of the battery safeguarding for you.  In any case, charging your battery to about 80% and discharging to about 20% will significantly extend your battery's lifetime, particularly if the battery is only sparingly used.  (LiPo long-term storage charge is typically 40-60% of maximum charge)