This is the final solution for anyone who finds this thread in future and also wants to extract playable sounds from XWCs with their proper filenames:
1st step) above script (darkness_xcd.bms) should be used on initial *.xwc archives;
2nd step) this BMS
by Bnnm from HCS64 forum should be used on either WAVEDATA bigfile or STRINGS file.
Big thanks to both Luigi & Bnnm for their work!
Oh and the last thing: resulting smaller XWCs contain 1 stream each and can be played back with VGMstream plugin for Foobar2000/Winamp/XMPlay.
Have a nice day everyone!