Going to Touchdown settings>notifications will give options for applying sounds. Choosing Email Ringtone will let you choose the application that contains the sound you want and and the particular sound or point to the file in media storage that in most cases will be default device sounds. You can have downloaded sounds show here by placing them in the appropriate media folder on the device. This is usually in a folder marked system/media/audio/notifications.
If you have sound files on an SD card, you can type in the path to the files in the Email custom sound section, but make sure to prefix the path with your device's SD card location. For example, /sdcard/ (/sdcard/myfolder/mysubfolder) or /storage01, (/storage01/myfolder/mysubfolder). Once you type in the path, the list of available sounds will be populated in the Select Sound list, from where you can now select the sound you want to use. If you have trouble getting the sound file to show, try going to Touchdown settings>advanced>legacy settings screen>advanced>Email alerts. Press Customize settings and select Play Sound and Custom Sound. Type the path to your sound here. It should allow you to select the sound if the path is correct.
Please make sure that you click the OK button and save settings to make sure your settings are applied.
Are you having trouble getting Touchdown to see the file? If so, get one of those free file managers from Google Play and navigate to the folder that contains your file. Note the path at the top in the file manager. Then use this path in Touchdown. Don’t forget to begin with an / in the path.