Sound files loaded into Cinema 4D via the Attribute Manager must have a format that Cinema can read (see Sound Formats). The file is opened by clicking on the button next to the file name field.
In the Attribute Manager, you will also find a setting called Probe. This defines how many samples will be used to calculate the amplitude — the greater the number of samples in this calculation, the more damped the sound will become. In other words, the sound gets quieter as you increase the Probe value.