Plain old oil based spray paint in a can -- sand, prime, sand, and paint. It will dry hard to a hard finish in a couple of weeks -- but it still might look "tacky" when fully dry. It will stink like spray paint for about 4 months.
For a clear coat -- there are several wipe on poly's you apply in thin coats with a cloth. If the speakers are a wooden veneer, this would be a very good choice. Wipe on finishes are easy to apply and get a good result. It will take about 10 coats, but looks very subdued compared to brushed on or sprayed finishes. Look at any big box hardware store for "Wipe on Polyurethane".. Minwax and many other companies make the same type of product.
