Fix no audio in output videos

[Baronsindo]

i removed '-an'
because of it the outputted videos has no sound

• If this is a bugfix, I have provided code that clearly demonstrates the problem and that works when used with this PR
• I have added a test to the test suite, if necessary
• I have properly documented new or changed features in the documention, or the docstrings
• I have properly documented unusual changes to the code in the comments around it
• I have made note of any breaking/backwards incompatible changes

[Overdrivr]

Thanks for your contribution, does this PR refers to #876 ?
Could you write a test case demonstrating the lack of audio in the exported file ?

[ThePhonon]

Note: the -an option is arguably needed. The bug is that it is not provided before the '-i' option.
See #968

[Baronsindo]

Thanks for clearing that out.

[tburrows13]

The general census seems to be that #968 should be merged, not this one.