Voice Memos are not included in iCloud Backup with iOS version 12 or later. Also note that you cannot view any contents of iCloud Backup; you can only use the backup to do a full restore of a device or to setup a new device.
Voice Memos can be synced with your other devices via iCloud. See instructions in See your recordings on all your Apple devices - Apple Support
While voice memos can be synced via iCloud, there is no option to view them at iCloud.com. That applies to voice memos and to some other things (e.g., iCloud Messages) that can be synced through iCloud but not viewed at iCloud.com.
Photos, Mail, Contacts, etc that you see at iCloud.com are synced and can also be viewed at iCloud.
Finally, some things such as photos can be either synced or included in Backup. If iCloud Photos is turned on, they will be synced. If iCloud Photos is not turned on, the photos can be included in iCloud Backup.
See: What does iCloud back up? - Apple Support