I can do that but after i've posted it i've noticed that there is a maybe deciding restriction.
The problem with my suggested method, if it works is, that the sound will appear suddenly in that moment when you touch the ground with the defined sound. The normal announcer/horn sounds get louder the closer you come.
Another thing is, that every mod with different settings in it's sfx file will overwrite the tracksettings and the normal sound of the underground will get lost.
But here is an example of what i mean. (But remember: never tested, don't know if it works

)
The tdf entries define the behaviour of the different types of underground/walls... when you touch them. Let's take a normal tdf entry for the gravel:
// Gravel
[FEEDBACK]
Dry=0.52 Wet=0.50 Resistance=5000.00 BumpAmp=0.052 BumpWavelen=4.0 Legal=false Spring=0.0 Damper=0.0 CollFrict=0.8 Sparks=0 Scraping=0 Sink=0.017 Sound=gravel
Reaction=dust Tex=Dustroad.tga Max=128 TopSpeed=104.0 Scale=(1.0,1.0,1.0) Growth=(4.0,3.0,2.0) ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
//Reaction=dirt Tex=Dustroad.tga Max=128 Scale=(0.5,0.5,0.5) Growth=(0.8,0.8,0.8) ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Reaction=softskid Tex=skidbrown.tga Max=1024 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Materials=grvl
The values you would have to change for the area where you want to hear the sounds, are:
Sound=gravel and
Materials=grvl
Copy and paste the whole entry and change e.g.
Sound=gravel into:
Sound=talking and
Materials=grvl into Materials=talkmody
The parts of the gmt(s) where you want to hear the sound on, have to have the materialname
talkmody too!
And the other values have to match the properties of the underground.
E.g. if it is road, you would have to take a normal "road" section, copy and paste it and change these both values.
Rename the sound file you want to use into talking.wav and put it into the folder: ..\GameData\Sounds\TMTires and ..\GameData\Sounds\Default
Maybe you additionally will have to rename and put the soundfile in the folders as listed here:
Into the folder ..\GameData\Sounds\TMTires:
talking_dry.wav
talkingskid_ex.wav
talkingskid_in.wav
talkingroll_ex.wav
talkingroll_in.wav
Into the folder ..\GameData\Sounds\Secondary
talkingroll_in.wav
talkingskid_in.wav
I hope i didn't forget a thing.