I think put_object_url is indeed what you want. If you follow the url method back to where it is defined, you can see it uses a similar method underlying it called get_object_url here (https://github.com/fog/fog/blob/dc7c5e285a1a252031d3d1570cbf2289f7137ed0/lib/fog/aws/models/storage/files.rb#L83). You should be able to do something similar and can do so by calling this method from the fog_s3 object you already created above. It should end up just looking like this:
headers = {}
options = { path_style: true }
url = fog_s3.put_object_url(bucket, object_path, expires, headers, options)
Note that unlike get_object_url there is an extra headers option snuck in there (which you can use to do stuff like set Content-Type I believe).
Hope that sorts it for you, but just let me know if you have further questions. Thanks!
Addendum
Hmm, seems there may be a bug related to this after all (I'm wondering now how much this portion of the code has been exercised). I think you should be able to work around it though (but I'm not certain). I suspect you can just duplicate the value in the options as a query param also. Could you try something like this?
headers = query = { 'Content-Type' => 'audio/wav' }
options = { path_style: true, query: query }
url = fog_s3.put_object_url(bucket, object_path, expires, headers, options)
Hopefully that fills in the blanks for you (and if so we can think some more about fixing that behavior within fog if it makes sense to do so). Thanks!