The Maestro Pro isn't actually a sound card - it lacks DA and AD converters. In a nutshell it's a purely digital audio interface, S/PDIF and Toslink.
To hear whatever you've recorded you'd need a DAC, e.g. Paula's which is directly supported by the Maestro software in pseudo 14 bit mode. Of course you can also run the output into your digital PA for HiFi output. AHI supports the Maestro Pro, so you could use that combination as 'sound card' as well.
For MP3 you'll need a beefy CPU since there's no DSP.
(There's also the earlier Maestro 'non-Pro' which is input only and lacks buffers, so it can record to memory only.)
