Here is a portable (I think) `filevalid_*' that uses
the `system_i' opcode (tested with 5.07)

<CsoundSynthesizer>
<CsInstruments>

gios_check = 0

;; ires filevalid_S filename
opcode filevalid_S, i, S
  Sfile xin
  if (gios_check == 1) igoto cont
  ires system_i 1, "exit nonzero"
  if (ires != 0) then
    ;; unix (like)
    gSfmt = "test ! -f '%s'"
  else
    ;; windows/dos
    gSfmt = "if exist %s exit 1"
  endif
  gios_check = 1
cont:
  Scmd sprintf gSfmt, Sfile
  ires system_i 1, Scmd
  xout (ires != 0 ? 1 : 0)
endop 

;; ires filevalid_i ifilcod
opcode filevalid_i, i, i
  ifilcod xin
  Sfile sprintf "soundin.%d", ifilcod
  ires filevalid_S Sfile
  xout ires
endop

instr 1   ; test
ires filevalid_S "somefile.wav"
print ires
ires filevalid_i 1     ; check soundin.1
print ires
endin

</CsInstruments>
<CsScore>
i1 0 1
</CsScore>
</CsoundSynthesizer>

tito