14 版上声音很好，但升级到 16 版后就没有声音了。
aplay 似乎找到了声卡：
$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC3235 Analog [ALC3235 Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 7: HDMI 1 [HDMI 1]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 8: HDMI 2 [HDMI 2]
Subdevices: 1/1
Subdevice #0: subdevice #0
根据其他帖子的建议，我重新安装了 alsa-base 和 pulseaudio
在 alsamixer （和 gnome-alsamixer）上，扬声器没有静音。
在系统设置-->音频中，“播放声音”框是空白的，并且没有选中静音框。
$ uname -r
4.4.0-139-generic
$ find /lib/modules/$(uname -r) | grep snd
显示大量驱动程序
======== 以下是 dmesg 显示的内容：
$ dmesg | grep -iE 'snd|hda'
[ 5.291111] snd_hda_intel 0000:00:1f.3: bound 0000:00:02.0 (ops i915_audio_component_bind_ops [i915_bpo])
[ 5.375131] snd_hda_codec_realtek hdaudioC0D0: autoconfig for ALC3235: line_outs=1 (0x16/0x0/0x0/0x0/0x0) type:line
[ 5.375139] snd_hda_codec_realtek hdaudioC0D0: speaker_outs=1 (0x14/0x0/0x0/0x0/0x0)
[ 5.375142] snd_hda_codec_realtek hdaudioC0D0: hp_outs=1 (0x15/0x0/0x0/0x0/0x0)
[ 5.375144] snd_hda_codec_realtek hdaudioC0D0: mono: mono_out=0x0
[ 5.375146] snd_hda_codec_realtek hdaudioC0D0: inputs:
[ 5.375148] snd_hda_codec_realtek hdaudioC0D0: Headphone Mic=0x18
[ 5.375150] snd_hda_codec_realtek hdaudioC0D0: Dock Mic=0x19
[ 5.375153] snd_hda_codec_realtek hdaudioC0D0: Headset Mic=0x1a
[ 5.375155] snd_hda_codec_realtek hdaudioC0D0: Internal Mic=0x13
[ 5.423487] input: HDA Intel PCH Headphone Mic as /devices/pci0000:00/0000:00:1f.3/sound/card0/input11
[ 5.423550] input: HDA Intel PCH Dock Mic as /devices/pci0000:00/0000:00:1f.3/sound/card0/input12
[ 5.423604] input: HDA Intel PCH Dock Line Out as /devices/pci0000:00/0000:00:1f.3/sound/card0/input13
[ 5.423658] input: HDA Intel PCH HDMI/DP,pcm=3 as /devices/pci0000:00/0000:00:1f.3/sound/card0/input14
[ 5.423694] input: HDA Intel PCH HDMI/DP,pcm=7 as /devices/pci0000:00/0000:00:1f.3/sound/card0/input15
[ 5.423729] input: HDA Intel PCH HDMI/DP,pcm=8 as /device/pci0000:000000:00:1f.3/sound/card0/input16