You can use https://www.mp3smaller.com/ to compress your songs(lower file size), 80kbps is good enough, talking with experience.
Also if you want to download a song from youtube, i use this: https://www.flv2mp3.by/tr16/
If your song volume is too low in map, you can use http://www.kanssoftware.com/download_tr.htm , 1.5 - 4.5 dB normalizing is enough.
In my opinion map being so plain is not a problem, just dont make it annoying and people would be grateful.You will start making better maps as you keep doing them.
For suggestion i would say start simple and make something new for your next maps since mta map editor is kinda cancer, wouldnt be good if you would say screw this and quit.
Good luck.