Hello,
is there a version for older IDEs?
No, currently you need at least Arduino 1.0 IDE.
are we free to do whatever we want during the audio playing? any restrictions?
It depends... You have to call the worker()-function frequently enough to avoid buffer underruns. This function reads the next sector from SD card into a ringbuffer to keep audio playing running.
The library uses timer1 on most Arduinos (timer5 on mega arduino). This should be no issues for standard libraries, but libraries which uses those timers may not work correctly. analogWrite() functions should work even with that modified timer. The timer's interrupt is used to for audio playback, disabling interrupts for more than 10 micro seconds could result in playback distortions. In general, very much of the available computing power of the Arduino is used during playback...
what about having other components on the SPI bus?
The lib could work with other stuff on the SPI bus as long as it has its own chip-select and does not need to change the spi transfer parameters (SD lib uses full speed). It seems to work with the Ethernet-Shield - also an example to start playback via webbrowser is included in the examples. For other SPI stuff it depends - just try it...