The MediaTrackConstraints dictionary's sampleSize property is a ConstrainULong describing the requested or mandatory constraints placed upon the value of the sampleSize constrainable property.
If needed, you can determine whether or not this constraint is supported by checking the value of MediaTrackSupportedConstraints.sampleSize as returned by a call to MediaDevices.getSupportedConstraints(). However, typically this is unnecessary since browsers will ignore any constraints they're unfamiliar with.
If this value is a number, the user agent will attempt to obtain media whose sample size (in bits per linear sample) is as close as possible to this number given the capabilities of the hardware and the other constraints specified. Otherwise, the value of this ConstrainULong will guide the user agent in its efforts to provide an exact match to the required sample size (if exact is specified or both min and max are provided and have the same value) or to a best-possible value.
Note: Since this property can only represent linear sample sizes, this constraint can only be met by devices that can produce audio with linear samples.