Methods and apparatus for hydrocarbon monitoring are provided. A method that may be performed by a flowmeter or monitoring system includes receiving downhole measurements of a flowing fluid from a flowmeter; determining a standard phase fraction of the flowing fluid based on the downhole measurements from the flowmeter; receiving surface measurements of the flowing fluid; determining a surface phase fraction of the flowing fluid based on the surface measurements; comparing the standard phase fraction to the surface phase fraction; based on the comparison being greater than a predetermined threshold, using the surface measurements as a reference to adjust a speed of sound (SoS) of a first phase until a target value is achieved; and receiving additional downhole measurements of the flowing fluid from the flowmeter, wherein the flowmeter is operating using the adjusted SoS of the first phase.