When audio for an application is too loud or too soft, Windows' volume mixer can be used to adjust the volume on a per-application basis. The volume mixer can be found by right-clicking the speaker icon in the taskbar and selecting Open Volume Mixer.
Change the overall speaker level by using the device slider underneath "Device." This sets the master volume level. The line extending from the device slider indicates the maximum volume, which no application can exceed.
Volume for each application can be independently adjusted by using the slider underneath the desired application.