I tried both of these and neither of them is any good for this task (I know they were only suggestions but I felt I should point this out for the benefit of others). The first one can convert a large number of files at once, but even at highest quality settings the converted files have a very noticeable reduction in quality over the originals (an audible "hissing" that is annoying particularly in the quieter tracks). I've been using the file Catfish_Muse in particular for testing the audio quality. It has some passages of silence after or before a fading in/out instrument.
The second one doesn't seem to allow conversion of a large number of files at once, or at least doesn't make it very easy to do so with its interface.
After a short amount of searching, I found this one, tested it and it appears to work very well.
Free Mp3/Wma/Ogg Converter
Note that if you install this program, you do not need to install the "RelevantKnowledge" thing it asks you about at the end - you can safely decline.
Using this program,
1. search for *.ogg in
Steam\steamapps\common\sid meier's civilization v\Assets\Sounds\Streamed\Music in Windows Explorer as I described earlier and then select all files (ctrl+A).
2. With
Free Mp3/Wma/Ogg Converter opened, drag and drop all of those ogg files onto the interface.
3. Press "Next".
4. Choose the destination folder, make sure "mp3" is selected, and under "presets" choose the quality setting:
"For Media Player - MP3: 48000 Hz, Stereo, 128kbps" (the second one).
You could choose a higher quality setting if you want, but the originals are about 128kbps and you can't magically get higher quality from a conversion process.
5. Press "Convert", wait maybe 40 minutes or so (depending on your cpu).
6.
Enjoy the game's entire soundtrack in MP3 that is about the same quality as the originals. (I couldn't detect any degradation in quality over the originals. If you have good ears and can tell the difference, please do tell.) The converted files tend to be around about the same size as the originals but vary between being larger or smaller. I guess this is because ogg and mp3 use different compression algorithms.
All up it's just over 1000MB worth of mp3 music, or just under 1GB. 307 files. 18hrs20mins playback time.
I hope this will be of use to you, thadian.
Hopefully this is not infringing on copyright or anything like that, considering you need to have the original files to follow the instructions in the first place.