testing shows that on an SB16, there really is no "highspeed DSP" mode,
the DSP responds to the command the same way it does a normal
non-highspeed command. updated sample rate limit code to reflect SB16
behavior. It seems the SB16 allows all playback commands to run at a
time constant from 4KHz up to the max supported by the DSP (48KHz on the
ViBRA card I tested) with or without hispeed DSP commands.