I suppose this is a workable solution which has some practical values. The upper plot is a mini band scope, and the lower plot is an aid for CW tuning. The audio output is band-pass filtered with the parameters adjustable by the sliders before being sent to the USB audio output device, Zoom G3.
Note that the card and the device numbers may change, and now Zoom G3 is hw:1,0.
user1@UNIX [03:39:45] [~/Music] % aplay -l **** List of PLAYBACK Hardware Devices **** card 0: MID [HDA Intel MID], device 0: ALC889 Analog [ALC889 Analog] Subdevices: 1/1 Subdevice #0: subdevice #0 card 0: MID [HDA Intel MID], device 3: ALC889 Digital [ALC889 Digital] Subdevices: 1/1 Subdevice #0: subdevice #0 card 1: Series [ZOOM G Series], device 0: USB Audio [USB Audio] Subdevices: 0/1 Subdevice #0: subdevice #0 card 2: CODEC [USB Audio CODEC], device 0: USB Audio [USB Audio] Subdevices: 1/1 Subdevice #0: subdevice #0 card 3: HDMI [HDA ATI HDMI], device 3: HDMI 0 [HDMI 0] Subdevices: 1/1 Subdevice #0: subdevice #0