> Unfortunately, I cannot count on python always being available.
> 
> Can anyone suggest a simple solution using the system opcode - one that perhaps parallels "os.path.exists"?

The hidden filevalid

<CsoundSynthesizer>
<CsInstruments>
sr     = 44100
ksmps  = 64
nchnls = 1

;; filevalid for csound < 5.13 (tested with 5.07)
opcode filevalid, i, S
  Sfile xin
  ires ftgen 2012, 0, 2, 1, Sfile, 0, 0, 0
  xout ires - 2011
endop

instr 1
Sfile = "dindondan.wav"
ires filevalid Sfile
if (ires != 1) igoto end
aout soundin Sfile
out aout
end:
if (ires != 1) then
  turnoff
endif
endin
</CsInstruments>
<CsScore>
i1 0 1
e
</CsScore>
</CsoundSynthesizer>

tito