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Frames are 16bit (65,536 byte frame size).
Audio is referenced as Frame number + offset in sample count.
Audio samples are 4bit ADPCM (i.e. 512 bytes = 1024 samples).
All calculated offsets for audio records are in reference to the beginning of the audio data which is at 0x8000 in the .mth file.

//1st audio record
0x0100: 01 // Frame (Start)
0x0101: 00 00 // Offset sample count into frame for start of audio data (16bit big endian)
0x0103: FF FF FF // majority of records defined this way. when not, references a frame + offset as above. unknown, maybe loop data
0x0106: FF FF FF // majority of records defined this way. when not, references a frame + offset as above. unknown, maybe loop data
0x0109: 00 00 00 // majority of records defined this way. when not, pattern is apperent but haven't analyzed it yet.
0x010C: 01 // Frame (End)
0x010D: CE 79 // Offset sample count into frame for end of audio data (16bit big endian)
0x010F: 0B // not known yet. seems to be a bitmask or referenced in nibbles. probably sample rate and other flags.
//result: start @ 0; end @ 52875; 52875 samples

//2nd audio record
0x0110: 01
0x0111: CE 7A
0x0113: FF FF FF 
0x0116: FF FF FF 
0x0119: 00 00 00 
0x011C: 02 // second frame, so add 65,536 to offset
0x011D: F4 6B
0x011F: 0B
//result: start @ 52876; end @ 128107; 75231 samples

//3rd audio record
0x0120: 02
0x0121: F4 6C
0x0123: FF FF FF
0x0126: FF FF FF
0x0129: 00 00 00
0x012C: 03 // 3rd frame, so start at 65,536*2 this time (frame_size*(n-1))
0x012D: D5 03
0x012F: 0B
//result: start @ 128108; end @ 185603; 57495 samples

@echo off

if "%~1"=="" (goto error)
Set filename="%~1"
echo.Filename is: %filename%

Set Folder="%~dp1"
echo.Folder is: %Folder%
Set Name="%~nx1"
echo.Name is: %Name%

rem Check that the extension is .vox
set Ext="%Name:~-4,-1%"
rem echo %Ext%
if not %Ext%=="vox" (goto exterror)

rem Strip the extension and replace it for Outname
set Outname="%Name:~1,-5%.ima"
echo.Outname is: %Outname%

set SampleRate="none"
If NOT %Name%==%Name:5512=% (set SampleRate="-r 5512")
If NOT %Name%==%Name:11025=% (set SampleRate="-r 11025")
If NOT %Name%==%Name:22050=% (set SampleRate="-r 22050")
if %SampleRate%=="none" (goto samperror)
echo.SampleRate is: %SampleRate:~1,-1%

rem echo.Current Folder is:
rem
cd /d %Folder%
rem
pause

echo "C:\Program Files (x86)\sox-14-4-2\sox" %SampleRate:~1,-1% %Name% -e ima-adpcm -c 1 %Outname%
"C:\Program Files (x86)\sox-14-4-2\sox" %SampleRate:~1,-1% %Name% -e ima-adpcm -c 1 %Outname%
rem
echo Import this file into a sound file using ADPCM as a 4 bit ADPCM clip
rem
pause
goto end
:samperror
Echo You must have the sample rate as part of the filename
pause
goto end
:exterror
Echo You can only use *.vox files
pause
goto end
:error
Echo You did not enter the input file name.
pause
:end

So happy to have found this thread - and really appreciate the work done to decipher the MTH sound files and assemble a program to manipulate the sounds inside.

I have a PS2 steamer pulling my Polar Express around our Christmas village.  Not only were the sounds not really appropriate for the Polar Express, but the command environment made it so the conductor announcement car was not functional.  This is no longer an issue!

Following the directions above, I was able to use ADPCM, Audacity, and sox to successfully replace all of the engine sounds this afternoon.  The only caveat, I was never able to export from Audacity to vox format for sox conversion.  Instead, when exporting from Audacity, I set "Save as type" to "(external program)".  The command was specified as

"C:\Program Files (x86)\sox-14-4-2\sox.exe" -t wav - -e ima-adpcm -c 1 "%f"

allowing Audacity to pipe its wav output directly into sox, skipping the intermediate vox step.

Thanks again!
-Sam

@echo off
rem Updated 1 Mar 2018
if "%~1"=="" (goto error)
Set filename="%~1"
echo.Filename is: %filename%

Set Folder="%~dp1"
echo.Folder is: %Folder%
Set Name="%~nx1"
echo.Name is: %Name%

rem Check that the extension is .raw
set Ext="%Name:~-4,-1%"
echo %Ext%
if not %Ext%=="raw" (goto exterror)

rem Strip the extension and replace it for Outname
set Outname="%Name:~1,-5%_be.raw"
echo.Outname is: %Outname%

set SampleRate="none"
If NOT %Name%==%Name:5512=% (set SampleRate="-r 5512")
If NOT %Name%==%Name:11025=% (set SampleRate="-r 11025")
If NOT %Name%==%Name:22050=% (set SampleRate="-r 22050")
if %SampleRate%=="none" (goto samperror)
echo.SampleRate is: %SampleRate:~1,-1%

rem echo.Current Folder is:
rem
cd /d %Folder%
rem
pause

echo "C:\Program Files (x86)\sox-14-4-2\sox" -t .s24 -c 1 %SampleRate:~1,-1% -B %Name% -c 1 %Outname%
"C:\Program Files (x86)\sox-14-4-2\sox" -t .s24 -c 1 %SampleRate:~1,-1% -B %Name% -c 1 %Outname%
rem
echo Import this file into a sound file using ADPCM as a 24 bit signed big-endian clip
rem
pause
goto end
rem
:samperror
Echo You must have the sample rate as part of the filename
pause
goto end
:exterror
Echo You can only use *.raw files
pause
goto end
:error
Echo You did not enter the input file name.
pause
:end

@echo off

if "%~1"=="" (goto error)
Set filename="%~1"
echo.Filename is: %filename%

Set Folder="%~dp1"
echo.Folder is: %Folder%
Set Name="%~nx1"
echo.Name is: %Name%

rem Check that the extension is .ima
set Ext="%Name:~-4,-1%"
echo %Ext%
if not %Ext%=="ima" (goto exterror)

rem Strip the extension and replace it for Outname
set Outname="%Name:~1,-5%.vox"

echo.Outname is: %Outname%
rem echo.Current Folder is:
rem
cd /d %Folder%
rem

echo "C:\Program Files (x86)\sox-14-4-2\sox" %Name% -e oki-adpcm -c 1 %Outname%
"C:\Program Files (x86)\sox-14-4-2\sox" %Name% -e oki-adpcm -c 1 %Outname%
rem
echo Import this file into Audacity as VOX-ADPCM
echo Select Import -- Raw Data
echo Pick the .vox file created by this batch file
echo Select Encoding: VOX-ADPCM
echo Select Byte order: Big-Endian
echo Select Channels: 1
echo Select Sample rate: 5512/11025/22050 as necessary
echo Press [Import]
rem
pause
goto end
rem
:exterror
Echo You can only use *.ima files
pause
goto end
:error
Echo You did not enter the input file name.
pause
:end