When connecting remotely via Splashtop Streamer, you have the option to define the sound output settings.

Open the Splashtop Streamer and go to the Settings tab > Sound.

You can also change these settings remotely by tweaking a few registry keys via scripting.
The AutoMute key related to this setting is located under:

 HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Splashtop Inc.\Splashtop Remote Server

There are three possible values for it:

• 0 - Output sound both over the remote connection and on this PC
• 1 - Output sound over the remote connection only
• 2 - Output sound on this computer only

The command:

wmic os get osarchitecture | find "64-bit" && reg add "HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Splashtop Inc.\Splashtop Remote Server" /v AutoMute /t REG_DWORD /d X /f || reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Splashtop Inc.\Splashtop Remote Server" /v AutoMute /t REG_DWORD /d X /f
net stop "SplashtopRemoteService" && net start "SplashtopRemoteService"

Please ensure to change the value of the X variable. 

Note: If you're on a call with a customer using Microsoft Teams and you then connect remotely via Splashtop to the same device, you may hear an echo at your end. To fix this, please ensure that "Output sound on this computer only" is selected (if this change is performed while you are connected via Splashtop, you will need to close the connection and reconnect afterward for the change to take effect).