You can check the condition in SetProperty itself. You can use the below condition in your requirement.
<Property Id="SOUNDSERVER">
<RegistrySearch Id="SoundServer"
Root="HKLM"
Key="[APPLICATIONHIVE]"
Name="SoundServer"
Type="raw"
Win64="yes" />
</Property>
<Property Id="WIXUI_SOUNDSERVER" Value="1" />
<SetProperty Id="WIXUI_SOUNDSERVER" After="AppSearch" Value="{}">
SOUNDSERVER="#0"
</SetProperty>
<Control Id="SoundServerCheck" Type="CheckBox" X="20" Y="148" Width="80" Height="10" Property="WIXUI_SOUNDSERVER" CheckBoxValue="#1" Text="Sound Server"></Control>
Edit:
Remove WIXUI_SOUNDSERVER property and use the SOUNDSERVER property in all places to uncheck the checkbox while registry does not exist.
<SetProperty Id="SOUNDSERVER" After="AppSearch" Value="{}">
(SOUNDSERVER="#0")
</SetProperty>
<Control Id="SoundServerCheck" Type="CheckBox" X="20" Y="148" Width="80" Height="10" Property="SOUNDSERVER" CheckBoxValue="#1" Text="Sound Server"></Control>