Go to Google and type in the words with double quotes, for example "car audio" and check the results, and then "car audios" and check these. This will give a good indication of the popularity of each phrase.
Then go to Google's old Adwords site:
https://adwords.google.com/AdWords/Welcome.html
- click on 'create and preview ads'
- then next page click 'continue'
- in the exact phrase match box, type for example, car audio on one line, and car audios on the next line, and the results will show how often the term is searched for on Google each day/week/month.
From this analysis you should be able to work out whether you want to reg that name that ends in audios.
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