Onn is Walmart's bargain basement brand made in China at the lowest possible price point. If they are older than 3 years, they have probably failed. You can test the speakers on another device.
You said you connected them to 'Speaker Out'. The word 'Out' concerns me. Speaker outputs normally say 'speaker left' and 'speaker right', with no mention of 'OUT' which is normally reserved for an Auxilliary 'OUTput.
You haven't included a model number. Google
make model manual
Some other ideas here
https://www.google.com/search?q=Onn+mini+stereo++speakers+not+working
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