How can I recover iTunes music if my previous iMac is no longer accessible?

From your Time Machine backup of the old Mac! Check your other devices and see if your music is available on iCloud if it isn't on iCloud and you never backed up the old computer using Time Machine or another method of backup then the music is likely gone.

When your computer goes bad and you lose your iTunes music library, especially if it contained songs imported from CDs or other sources and you never had subscribed to iTunes Match then this is a problem because, unlike purchased music which can often be re-downloaded from the iTunes Store, music you've imported yourself is usually only stored locally on your computer.

If you subscribe to Apple Music, you can make your entire music library available on all your devices when you’re signed in to the same Apple Account and you turn on Sync Library in Settings. You can access your library any time you’re connected to the internet, or even offline, if you download your music.

Here's a breakdown of why this happens and what you can do about it:

Why it happens:

• Local Storage: When you import music from CDs or other non-iTunes Store sources, iTunes stores these files directly on your computer's hard drive within the iTunes Media folder.
• No Cloud Backup (by default): Unlike purchased music (which is tied to your Apple ID and can be re-downloaded), or music synced through services like Apple Music or iTunes Match (which stores your library in the cloud), your self-imported music isn't automatically backed up to Apple's servers unless you specifically use one of those subscription services.

Re: “I have enabled ‘sync across devices’ enabled on my iPhone but I don’t think the setting ‘sync across devices’”

Apple only supports iCloud-based synchronization of Music libraries if you have a subscription to iTunes Match, or a subscription to the Apple Music streaming service.

Use Sync Library with your Apple Music subscription - Apple Support

iTunes Match / Apple Music / Apple One an… - Apple Community

if you are not using either subscription service, or if you are a subscriber to Apple Music who keeps Sync Library turned off, there is manual synchronization. For music from CDs, this is one-way: from a computer to an iPhone, iPad, or iPod.

in any event, it is a good idea to protect and back up the library of purchased music on your computer. Assuming that you’ve kept your CDs, there is the option of re-importing music from CD, but this can be time-consuming.