Iphone says songs are on it, but when I hit play it says I need to download the song

Question

So I had a few strange things happen every since itunes 10 and now more problems since updating to iOS 7 and iOS 8. At first I just started having about 20% of my songs play part way through then go silent for the last part of the song. They work fine on my computer and iPad but not my iPhone nor iTouch. Then after the iOS 7 update which failed the first time on my phone causing me to restore from an update a week prior due to the current back up being corrupted for who knows why.
So after the restore when songs were added to my iPhone 5s, it says I have X amount of music on my phone but shows I still have 15 GB of space left while iTunes says I should have 3 GB of space. I then go to play a song and it shows the little download icon next to it and skips to the next song. This has happened on music I bought and some i Imported from CDs (confusing how I download a song from a CD as it's asking me to do). It was about 250 songs that this happened to out of the 3,500 I was putting on my device.
Then when iOS 8 came out I noticed that music would not sync to my phone because it said I had no space left, it said I needed an additional 28 GB of storage yet I don't even have 28 GB of music. I was trying to add 15 new songs so it wouldn't be that much space. I then tried removing the music by unchecking the sync music box and the phone would sync. After the sync iTunes said the phone had no music on it and about 20 GB of free space. Then I checked it on the phone itself and all of the music was on the phone.
In a fit of unhappiness as often happens to me with Apple products, I manually removed all the music on the phone through the settings interface. Then I synced it back up to my computer. It says it put on about 3600 songs on to my phone and I only have 3 GB free as I would expect. However when I check my phone it says I have 15 GB of free space (very confused how they are so far off). I did notice when it was syncing music it said it was syncing up to 960 songs. Then when I went to play music on my phone it says I have to redownload 2500 songs! Even ones that I imported from CDs I own! Why can my phone not sync with iTunes??? I played 5 songs on shuffle and it went through 117 before it could find 5 that it didn't need to download.

Answer 1

Was the music purchased via your iTunes account and/or did you buy it from other sources or copy it from your own CDs, or did the music belong to the previous owner ? If the music was yours then you can sync it back to the iPod via your computer's iTunes. If the music is not still on your computer then have you got a backup of it ? If not then for iTunes purchases you might be able to re-download it depending on which country you are in. If the music was put on the iPod by the previous owner then you will need to buy your own copies of it.