Internet Protocol Suite, TCP and IP, is bunch of communications protocols on PC and all over Internet. The TCP or IP suite is termed for 2 most vital protocols in it that is Transmission Control Protocol and Internet Protocol.

PC must use communications protocol each time it connects to Internet. As there are various applications and services competing for communications channels, communications protocol is classified in port numbers.

Dilemma
Application can use various port numbers, but as so many applications and services require ports, ports must be allotted in a structured method.

The issues happen when one program uses a port that a different application suppose to be available.

IANA
To keep things arranged Internet Assigned Numbers Authority allocates port numbers and manage registry of working and blocked port numbers. IANA even states process by which port numbers are requested. For present list of allotted port numbers for TCP and UDP, see reference link.

Port 3389
IANA has allotted port number 3389 to Windows Remote PC Connection program. Originally, both host as well as client PC hears on port from TCP. Meaning when 2 PCs are connected, information sent by them is delivered to port 3389 on other PC.

Remote PC Audio
Remote PC Audio may use UDP if present. UDP is first protocol of option for application's streaming audio. If UDP is unavailable, Remote PC uses default port, 3389 instead of TCP.

As a note, Remote Desktop can stream audio, but not MIDI sound.

Changing Port Numbers
The port number for Remote PC is not important if it is functioning unless Remote PC cannot connect or there is some other network issue.

A port number requires to be modified on one PC; it must be modified on others with which it interacts. In certain network cases, one might require changing port number. Here, port number 3390 is used. Have your network admin help you if you think you need to change a port number.