Sound from external DAC not detected; can't unmute devices in pavucontrol

Question

Recently I installed Linux Mint 17.1 KDE on a brand new computer. So far, only this problem. Using Audacity, I'm trying to capture audio streams from an external DAC-tuner connected by optical cable to a Xonar D2/PM sound card's S/PDIF input. But there's no sound input detected by Audacity or by headphones plugged into the Xonar's headphone output jack. However, music files played on the computer can be captured by Audacity and are heard through the headphones.

In PulseAudio Volume Control it's not possible to unmute the Xonar card, the onboard audio, or the graphics card – clicking on the red X beside the speaker icon associated with each of these devices doesn't remove the X. In KDE Mixer the sound devices can be unmuted/muted, and opening alsamixer in the terminal shows that nothing seems to be muted.

I want to record radio streams from the external DAC-tuner as I had been doing using Audacity and the same Xonar sound card in my previous old computer running Debian XFCE, with only ALSA and no PulseAudio. Then, whenever the sound system was playing and the computer was also on, the sound was always audible through the headphones connected to the Xonar. I've checked and rechecked the optical cable and headphone connections several times and they seem fine. Would greatly appreciate any helpful advice. I would think that sound must be audible from the headphones if it's being processed by the Xonar and before it has any possibility of being captured by Audacity.

Some relevant information:

cat /proc/asound/cards

0 [D2             ]: AV200 - Xonar D2
                  Asus Virtuoso 200 at 0xd000, irq 16
1 [HDMI           ]: HDA-Intel - HDA Intel HDMI
                  HDA Intel HDMI at 0xf7930000 irq 49
2 [NVidia         ]: HDA-Intel - HDA NVidia
                  HDA NVidia at 0xf7080000 irq 17
3 [SAA7134        ]: SAA7134 - SAA7134
                  saa7133[0] at 0xf7800000 irq 17

inxi -Fxz

System:    Host: desktop Kernel: 3.13.0-37-generic x86_64 (64 bit,    gcc: 4.8.2) 
           Desktop: KDE 4.14.2 (Qt 4.8.6) Distro: Linux Mint 17.1   Rebecca
Machine:   System: Gigabyte product: Z97-D3H
           Mobo: Gigabyte model: Z97-D3H-CF version: x.x Bios:   American Megatrends version: F7 date: 06/17/2014
CPU:       Quad core Intel Core i7-4790 CPU (-HT-MCP-) cache: 8192 KB      flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx) bmips: 28800.2 
       Clock Speeds: 1: 800.00 MHz 2: 800.00 MHz 3: 800.00 MHz 4:     800.00 MHz 5: 800.00 MHz 6: 800.00 MHz 7: 3601.00 MHz 8: 800.00 MHz
Graphics:  Card-1: Intel Xeon E3-1200 v3/4th Gen Core Processor      Integrated Graphics Controller bus-ID: 00:02.0 
           Card-2: NVIDIA GF119 [GeForce GT 610] bus-ID: 01:00.0 
           X.Org: 1.15.1 drivers: nvidia,intel (unloaded:     fbdev,vesa,nouveau) Resolution: 1280x1024@60.0hz 
           GLX Renderer: GeForce GT 610/PCIe/SSE2 GLX Version: 4.4.0  NVIDIA 331.113 Direct Rendering: Yes
Audio:     Card-1: Intel Xeon E3-1200 v3/4th Gen Core Processor HD   Audio Controller driver: snd_hda_intel bus-ID: 00:03.0
           Card-2: C-Media CMI8788 [Oxygen HD Audio] driver:  snd_virtuoso port: d000 bus-ID: 04:01.0
           Card-3: NVIDIA GF119 HDMI Audio Controller driver:  snd_hda_intel bus-ID: 01:00.1
           Sound: Advanced Linux Sound Architecture ver: k3.13.0-37- generic
Network:   Card: Intel Ethernet Connection I217-V driver: e1000e ver:  2.3.2-k port: f080 bus-ID: 00:19.0
           IF: eth0 state: up speed: 100 Mbps duplex: full mac:  <filter>
Drives:    HDD Total Size: 1000.2GB (0.7% used) 1: id: /dev/sda model: ST1000DX001 size: 1000.2GB 
Partition: ID: / size: 28G used: 6.6G (26%) fs: ext4 ID: /home size: 875G used: 408M (1%) fs: ext4 
           ID: swap-1 size: 16.00GB used: 0.00GB (0%) fs: swap 
RAID:      No RAID devices detected - /proc/mdstat and md_mod kernel        raid module present
Sensors:   System Temperatures: cpu: 29.8C mobo: 27.8C gpu: 0.0:36C 
       Fan Speeds (in rpm): cpu: N/A 
Info:      Processes: 221 Uptime: 1 min Memory: 895.0/15909.2MB   Runlevel: 2 Gcc sys: 4.8.2 Client: Shell inxi: 1.8.4 

Answer 1

Following a suggestion at another forum, I added a .asoundrc file and configured it. That caused the sound from the headphones to be detected but Audacity still wouldn't record the audio. Following a suggestion from an Audacity Forum Staff Member, I removed .asoundrc. The headphones continued outputting audio but Audacity continued not recording it. As no one has come up with a fix to the problem of Audacity not recording from an external DAC-tuner, I tried ocenaudio. Without any set-up, 2 mouse clicks and it started recording the external audio. Ocenaudio has all the capabilities I require, so goodbye Audacity.