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If you have a non-smartphone which has built-in MP3 playback capabilities you will generally be limited to the software built into the phone. Depending on the phone and how much effort the manufacturer decided into putting into software development for the non-phone capabilities it may be pretty decent software or just a plain music player like a cheap stand-alone MP3 player.
I especially enjoyed my Nokia 5230. It had an excellent MP3 player and I loaded the memory card with almost 24 hours of music covering dozens of different albums – music I transferred from CDs and cassettes, music I downloaded, and even some music I created myself. It was nice to have music on the go – in the car, while walking down the street, and even inside my house.
For the car I used a cassette adapter. When on the go I used the same stereo headset I used with the phone to talk. Inside my house I have portable stereos with line input jacks in several rooms. Simple adapter cables with 3.5 mm plugs permitted me to plug into any of these stereos. As a bonus when the phone was plugged into the stereo it was an excellent quality speakerphone with extremely high quality echo cancellation. I wish my current phone had as decent sound quality.