Problem syncing voice memos to iTunes in El Capitan

I would like to transfer voice memos recorded on my iPhone 5S to iTunes (12.3).

I select 'Music' in the sidebar and then select 'sync voice memos' but when I apply, I get the following error message:

The iPhone “XXX’s iPhone 5s” is synced with another iTunes library on “XXXX”. Do you want to erase this iPhone and sync with this iTunes library? An iPhone can be synced with only one iTunes library at a time.

I've only ever synced my iPhone to this iTunes library, so I don't know what to do.

The voice memos are also too big to email from my phone, so I'm stuck.

Any idea what I should do?

Thanks!