Refactor factory girl usage in specs

spec/models/sound_spec.rb

@@ -1,6 +1,6 @@
 describe Sound do
   subject     { sound }
-  let(:sound) { FactoryGirl.build(:sound) }
+  let(:sound) { build :sound }
 
   it { is_expected.to be_valid }
   it { is_expected.to belong_to :track }
@@ -19,7 +19,7 @@ describe Sound do
   end
 
   describe '#file=' do
-    let(:file) { FactoryGirl.attributes_for(:sound)[:file] }
+    let(:file) { attributes_for(:sound)[:file] }
 
     it 'saves the file SHA256 digest' do
       expect(sound.sha256).to eq Digest::SHA256.file(file.path).hexdigest