My work-in-progress Hornby APT-P fitted with 3 x 8Mb Loksound chips, each loaded with recordings of the sole remaining example at the Crewe Heritage Centre. Completely authentic sounds include the air conditioning, compressor, passenger doors, driver's door, power car horn and right away. In the absence of traction motor blower sounds I used the next best thing - the Class 91. My thanks go to the volunteers at Crewe for hosting the recording session.
Modifications to the model include front and rear lights, a reduction of the overscale gap above each bogie, authentic coloured interiors, weathered underframes and bogies, and smoked windows. The power car uses the mechanism from a Heljan 33. The speakers are the narrow-bodied bass reflex type from DCC Supplies, in the underframes facing downwards.
Further modifications - I've chopped up another Hornby APT to make two articulated carriages (one first class, one second). I've also added the destination blinds (visible in the side windows of the driving cars). They read London Watford Preston Glasgow on one side and the reverse on the other. Made up in Microsoft Word, printed out and reduced several times on the photocopier. Finally, added the second power car which hides a secret. The vast empty space inside is used as a loudspeaker enclosure, making a huge difference to the volume and bass.
You can buy a single version of the sound chip from Olivia's Trains.