I have a lot of MP3 files on my phone and I would like to re-size them to save space. (Note: I do not want to trim the audio file.) I know I may lose some quality, but it is fine. What is the best way to do this? Perhaps an app? (Free is preferred)
For doing the conversion on device itself, I would suggest searching the Google Play Store for audio conversion apps. Several results look like viable candidates: The File Converter and Convert Audio, for example. The features to look for are downsampling, changing bitrate and frequency, conversion to more efficient formats like OGG, AIC or FLACC. I recommend trying each app yourself on one or two MP3 files to see if you get the results you want. Play Store's 15-minute refund policy should give you enough time to figure out if the app is worth the price.
With that said, I would highly recommend doing the conversion on your PC instead. First, by copying your MP3s from the phone to the PC you are making a backup. Just in case something goes wrong during the conversion process, you will not lose the original file from your handset. Second, there are a lot more choices in terms of software on Windows/Mac/Linux for audio file manipulation (especially the free and open-source variety,) and may also be able to do this in batch (i.e. convert multiple files automatically.)