Further Googling revealed this... 2nd response down is from someone on the Google Help Forum. Looks like you need to convert your Audio CD to MP3 or other suitable format and then upload.
https://productforums.google.com/forum/#!topic/play/3tDJCacq_8M;context-place=topicsearchin/play/google$20play$20music
Quote:
Hi Lizzie,
Welcome to the Google Play Help Forum.
If your following the requirements for uploading music (see info below), you should be able to upload your own music. However this may require you changing the music format on the CD to one of the supported file types below. Try doing an internet or Google search on how to do this.
If you’re using Music Manager or Google Play Music for Chrome to upload music to your library, here are the types of files you can upload.
Note: Each music file can be up to 300 MB. When a file is converted to an MP3, the 300 MB limit applies to the converted MP3 file.
Supported file type Notes about supported file type
MP3 (.mp3)
AAC (.m4a) Files are converted to the same bitrate (quality) .mp3 files
WMA (.wma) Can be uploaded using the Windows version of Music Manager, not Google Play Music for Chrome
FLAC (.flac) 16 and 24-bit mono or stereo files supported Files are converted to 320 kbps .mp3 files
OGG (.ogg) Files are converted to the same bitrate (quality) .mp3 files
DRM protected AAC (.m4p) Some .m4p files can’t be uploaded Files are converted to the same bitrate (quality) .mp3 files
ALAC (.m4a) 16-bit files supported Files are converted to 320 kbps .mp3 files
Some music file types, including WAV (.wav), AIFF (.aiff), and RA (.r) can’t be uploaded to your library using Music Manager or Google Play Music for Chrome.
Alternatively if your having a problem using the Music Manager to upload, try using the Google Play Music for Chrome to upload.
Good luck,
JP