Use pd -help to find out.

As of Pd version 0.41.0:

 audio configuration flags:
  -r <n>           -- specify sample rate
  -audioindev ...  -- audio in devices; e.g., "1,3" for first and third
  -audiooutdev ... -- audio out devices (same)
  -audiodev ...    -- specify input and output together
  -inchannels ...  -- audio input channels (by device, like "2" or "16,8")
  -outchannels ... -- number of audio out channels (same)
  -channels ...    -- specify both input and output channels
  -audiobuf <n>    -- specify size of audio buffer in msec
  -blocksize <n>   -- specify audio I/O block size in sample frames
  -sleepgrain <n>  -- specify number of milliseconds to sleep when idle
  -nodac           -- suppress audio output
  -noadc           -- suppress audio input
  -noaudio         -- suppress audio input and output (-nosound is synonym)
  -listdev         -- list audio and MIDI devices
  -oss             -- use OSS audio API
  -32bit           ----- allow 32 bit OSS audio (for RME Hammerfall)
  -alsa            -- use ALSA audio API
  -alsaadd <name>  -- add an ALSA device name to list
  -jack            -- use JACK audio API
 MIDI configuration flags:
  -midiindev ...   -- midi in device list; e.g., "1,3" for first and third
  -midioutdev ...  -- midi out device list, same format
  -mididev ...     -- specify -midioutdev and -midiindev together
  -nomidiin        -- suppress MIDI input
  -nomidiout       -- suppress MIDI output
  -nomidi          -- suppress MIDI input and output
  -alsamidi        -- use ALSA midi API 
 other flags:
  -path <path>     -- add to file search path
  -nostdpath       -- don't search standard ("extra") directory
  -stdpath         -- search standard directory (true by default)
  -helppath <path> -- add to help file search path
  -open <file>     -- open file(s) on startup
  -lib <file>      -- load object library(s)
  -font-size <n>     -- specify default font size in points
  -font-face <name>  -- specify default font
  -font-weight <name>-- specify default font weight (normal or bold)
  -verbose         -- extra printout on startup and when searching for files
  -version         -- don't run Pd; just print out which version it is
  -d <n>           -- specify debug level
  -noloadbang      -- suppress all loadbangs
  -stderr          -- send printout to standard error instead of GUI
  -nogui           -- suppress starting the GUI
  -guiport <n>     -- connect to pre-existing GUI over port <n>
  -guicmd "cmd..." -- start alternatve GUI program (e.g., remote via ssh)
  -send "msg..."   -- send a message at startup, after patches are loaded
  -noprefs         -- suppress loading preferences on startup
  -rt or -realtime -- use real-time priority
  -nrt             -- don't use real-time priority
  -nosleep         -- spin, don't sleep (may lower latency on multi-CPUs)