You can´t call this function too often, but a buffer underrun
can occur when the time between two calls are too long.
As of Version 1.03: Add the flag <pre>SSDA_MODE_AUTOWORKER</pre>
to init and worker will be called automatically in background interrupt.
You don't need to call it yourself anymore. This makes audio playback more
robust when Serial functions are used. However, it does not work
in conjunction with Ethernet-Shield, as access to SPI bus
must be properly shared between SD-card and Ethernet chip.
==== play() ====