The source of this text is the (former) forum of GPS-Expert.nl
By using a Hex Editor we can modify the zūmo's Text To Speech (TTS) navigation voices to make them louder, this is handy if you feel the zūmo's navigation prompts are too quiet particularly when on the bike and wearing earplugs. This does not work on non-TTS voice files.
Increasing the volume from the original 75 setting to about 85 or 87 is a worthy increase in volume without any distortion creeping in (especially when using the car mount).
Download the following PDF file containing full instructions: HowTo - Upgrade volume of zūmo TTS voices.pdf
Already changed voices (for all zūmo/nüvi models which support TTS voices with filesizes from 2-4 MB):Like the zūmo 220
Already changed voices (for all zūmo/nüvi models which support TTS voices with filesizes from 12-14 MB:Like the zūmo 550, zūmo 660, zūmo 350 and Montana
Volume of non-TTS voices:I also have experimented on the normal voices. The British_English, Netherlands, Flemish, French, German and Spanish voices I already have them converted to have extra volume (normalising and the a 400% amplifying). These voices will be visible in your zūmo as version 2.21 (System/About - audio version)