Do I retain copyright to pervious source code used in a new project?

The Asker probably does retain ownership. However, this forum is not the place to examine the possibilities. You need to see Intellectual...

Can I legally print and sell a dog t-shirt that says "Hawaii Five-(dog paw with a circle around it) K-9 Unit?"

Probably but Hawaii 5-O is a registered trademark for hotel services. Your use for t-shirts probably does not infringe. Similar in sight and...

My father wrote many songs in 1933 and onward till his death in 1976.HE copywrited them all.HE also sang and co wrote some of

Who owns the copyright? Get counsel to research for ownership and assignment information via the Copyright Office. See what was disbursed from...

Rewrites may be copyright infringement. The Asker may be aware of the multitude of copyright infringement cases which have occurred in recent...

Lest copyright infringement is committed the Asker should be advised by Intellectual Property counsel.

Is it legal to sell things like shirts or posters of pop culture but transformed as animals (i.e. cats)?

This sounds like a derivative work which is a right belonging to the Copyright owner. If made without permission it may be copyright infringement....

If our company is using a streaming service's API or SDK that allows for commercial use, we don't have to pay royalties, right?

Read the agreement or Terms of Service to determine rights to download or listen to music. See Intellectual Property counsel.

I'm doing an art piece for my work, off the clock. Do I still own the rights to my work if I let them use it?

Being volunteered to do work for your employer means that the work is within your workshop. The Copyright will likely be owned by the Employer. ...

Creating a product based on TV show while making it unique enough and not calling it by name so I don't infringe on copyright

If the product even suggests the inspiring TV show it will be a derivative work and copyright infringement. See Intellectual Property counsel.

Can I use my given name as my business name?

You might not be able to use your name. The name may be in use for similar products of services. See Trademark Counsel.