Tuttut:
I also thought about calling the worker-function automatically in a timer interrupt. This should be possible, but only if that interrupt is set to non-blocking to guarantee the main audio interrupt the maximum priority. Maybe I will try to implement such in a next version of my lib, so that calling worker() is not necessary then anymore.
That would be awesome! 
Tuttut:
To your question about other Audio-Playback-Shields: As far as I have seen it, all other audio-shields contain a dedicated processor that reads the audio-data from storage and feed it to an audio-DAC. For a good sound the DAC must be feed with low jitter at the audio rate of the audio material, otherwise you may recognize audio distortions like slowdowns or stuttering. With a dedicated processor it doesn't matter what your Arduino does while audio is playing, but my library is for all those who don't want to spend extra money to such a thing. SimpleSDAudio get the most out of the AVR's by pushing the Arduino processor strongly to its limits - that's the price you have to pay if you want nice audio from that little thing.
Thanks for the info!
I think MP3 decoder shield might have such processor, but not WaveShield. From description they only have DAC with some basic low pass filters.
BTW, I'm just trying to understand how things work, not in any way implying anything bad about your library. In fact I think it's brilliant work, nobody else did anything like this with no external hardware (other than SD card)!