## Versions
- OS: Linux
- GTK+ Version: 3.22.28
- PyGObject Version: 3.26.1
- python-nbxmpp Version: 0.6.4
- Gajim Version: 1.0.0

## Traceback
```
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/gajim/config.py", line 3057, in on_play_button_clicked
    helpers.play_sound(snd_event_config_name)
  File "/usr/lib/python3.6/site-packages/gajim/common/helpers.py", line 856, in play_sound
    play_sound_file(path_to_soundfile)
  File "/usr/lib/python3.6/site-packages/gajim/common/helpers.py", line 929, in play_sound_file
    exec_command(command)
  File "/usr/lib/python3.6/site-packages/gajim/common/helpers.py", line 560, in exec_command
    p = subprocess.Popen(args)
  File "/usr/lib/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1344, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] Datei oder Verzeichnis nicht gefunden: 'aplay': 'aplay'

```
## Steps to reproduce the problem
...