Well I've produced a solution albeit not a very elegant one.
This topic
http://www.gscalecentral.net/m174807 shows plan A. I didn't solve the induced hum problem but, as my two mates with these locos were happy to go with battery power to the amplifier, for plan B that's what I did.
The AA cells are 2500mAh low self discharge so there shouldn't be a problem with lack of use over a period of several months. The track powered 5V relay switches the battery off when the track voltage falls below that required to power the sound chip. It is all too easy to leave a sound unit switched on if it has gone quiet, not normally a problem but in this case the battery would slowly discharge.
I feared that using an on board track powered charger would cause a return of the dreaded hum, hence the charging socket. This and the volume control are accessible via the water tank hatch on the rear truck.
The entire system is fitted into the rear truck. I opted to change the speaker for a Visaton FRS 8.
The crossing whistle and brake squeal are triggered by a sudden increase and decrease in track voltage respectively.
The Train Engineer system isn't responsive enough to do this, so I'll use diodes, either wired to a section of track or in the power supply leads. These may be shorted out by a switch when not required for a short section of track or remote controlled relay to trigger the sound with the diodes in the power supply leads.
I hope this block diagram explains the system. If you wish to build something similar the amplifier wiring diagram and data sheet may be downloaded from the net. I've not included it here as I'm unsure of copyright issues.
CV adjustment is a matter of personal taste. I chose to alter a few as follows.
CV
29 Set to 5 for Analogue permitted operation, 14 speed steps, LGB direction with directional lamps O.K.
Leave cab switch on NMRA.
139 Set to 200 for increased brake squeal volume.
153 Set to 3 for medium speaker.
193 Set to 20, bell on voltage.
194 Set to 45, bell off voltage.
195 Set to 3, Crossing Whistle sensitivity.
196 Set to 2, Brake squeal sensitivity.
197 Set to 31 to turn all available analogue sounds on.