Many thanks - yes, this did the trick.
Dapol have also sent me an expanded list of sound CVs for the 57xx, so I reproduce it all below for others.
(NB: changing the Overall Volume cv266 seems to reset other sound values, including cv584.
And lowest value is "1", a "0" being the equivalent of "255" or max.)
This is APPLICABLE TO THE ZIMO SOUND DECODERS ONLY!
Effect / CV No.
Overall volume = 266
F0 = 571
Fl = 514
F2 = 517
F3 = 520
F4 = 523
F5 = 526
F6 = 529
F7 = 532
F8 = 535
F9 = 538
F10 = 541
F11 = 544
F12 = 547
F13 = 550
F14 = 553
F15 = 556
F16 = 559
F17 = 562
F18 = 565
F19 = 568
F20 = 674
F21 = 677
F22 = 680
F23 = 683
F24 = 686
F25 = 689
F26 = 692
F27 = 695
F28 = 698
BOILING = 574
CHANGE DIRECTION = 576
BRAKE SQUEAL = 578
THYRISTOR = 580
START WHISTLE = 582
BLOW OFF (Draincocks) = 584
E-MOTOR = 586
DRIVING SOUNDS = 588
SWITCHGEAR = 590
2ND THYRISTOR = 492
PANTO UP = 594
PANTO DOWN = 596
PANTO STOP (DOWN) = 598
TURBO = 600
DYNAMICE BRAKE = 602
SWITCH INPUT 1 = 739
SWITCH INPUT 2 = 741
SWITCH INPUT 3 = 744
RANDOM 1 = 745
RANDOM 2 = 748
RANDOM 3 = 751
RANDOM 4 = 754
RANDOM 5 = 757
RANDOM 6 = 760
RANDOM 7 = 763
RANDOM 8 = 766