Some DSP commands I figured out:

Command byte(s)     Command                         comments
---------------------------------------------------------------------------------
1Ch                 Auto initialize                 Not availible on SB 1.0 cards
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48h,LSB,MSB         "Set block length"              Sets the block length that
                    LSB = Lower 8 bits of the       is played when commands
                    block length                    90h and 91h are sent. Pro
                    MSB = Higher 8 bits of the      only.
                    block length
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6Eh,hiver,lover     Set SB version (???)            Sets the DSP version bytes.
                    hiver = High version #          Strange.... Apparently on
                    lover = Low version #           Sound Blaster 16 cards.
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90h                 "High-speed 8 bits via DMA"     Plays sound via DMA at higher
                    Loop forever                    sampling rates. The sound
                                                    loops forever or until sound
                                                    card is reset. Apparently
                                                    not documented by Creative
                                                    Labs. Pro only.
--------------------------------------------------------------------------------
91h                 "High-speed 8 bits via DMA"     Like 90h, except the sound
                                                    card only plays the number
                                                    of samples specified when
                                                    command 48h was last sent.
                                                    At least this command is
                                                    documented. Pro only.
--------------------------------------------------------------------------------
98h                 "High-speed 8 bits via DMA"     Records sound via DMA at
                    Loop forever                    higher sampling rates. The
                                                    sound loops forever or until
                                                    sound card is reset.
                                                    Apparently not documented
                                                    by Creative Labs. Pro only.
--------------------------------------------------------------------------------
99h                 "High-speed 8 bits via DMA"     Like 98h, except the sound
                                                    card only records the number
                                                    of samples specified when
                                                    command 48h was last sent.
                                                    At least this command is
                                                    documented. Pro only.
--------------------------------------------------------------------------------
D1h                 "Turn on speaker connection"    Enables DSP output to the
                                                    amplifier. Pro only.
--------------------------------------------------------------------------------
D3h                 "Turn off speaker connection"   Disables DSP output to the
                                                    amplifier. Pro only.
                                                    Apparently the action of
                                                    sending command 1Ch
                                                    automatically turns on the
                                                    speaker.
--------------------------------------------------------------------------------
D8h                 "Ask speaker setting"           Returns the status of the
                                                    speaker connection. After
                                                    sending this command, read a
                                                    byte from the DSP. If the
                                                    value is 0, the speaker's
                                                    off. If the value is 255,
                                                    the speaker's on.
--------------------------------------------------------------------------------

Sources:
OUTER RIDGE readme file
The Sound Blaster Book
My own experimentation
Certain programs that I reverse-engineered