Here's one way to get better sound from your speakers that won't cost you a penny, get closer to them. The logic is easy to grasp, the closer you are to the speakers the more direct sound you'll hear. Close up listening minimizes the destructive effects of room reflections and reverberation that interfere with the sound. That's why sitting closer sounds clearer and less muddled. Also, remember to position the speakers with their tweeters at, or close to your ears' height when listening. And for optimum stereo imaging, sit equidistant from the left and right speakers.
If your speakers are more than 8 feet (2.4 meters) away right now, and you get a good deal closer the sound improvements won't be subtle. Give it a try.
Listen at close to or at realistic sound levels for the type of music you play, if your speakers or headphones can handle high volume without distorting. Experiment with playing music louder and see if it sounds better to you. Definitely apply common sense and avoid playing music so loud it disturbs other people, or endangers your hearing. In any case, avoid listening at high volume levels for more than 30 minutes at a time.
If you use your full-size headphones a lot and they are more than two years old, there's a good chance their ear pads are worn out or lost their cushy feel. Check if the manufacturer sells user-replaceable pads for your headphones. New pads do a better job sealing out external noise, and the headphones' bass will be optimized with fresh pads.
With in-ear headphones the same logic applies, so take the same course and order new ear tips for your headphones. If the original tips never provided a really good seal, you might be better off investing a few bucks in aftermarket replacement tips from another manufacturer. The difference in sound quality and noise isolation tight sealing tips provide are substantial, and tips are cheap!
If you play LPs, but the turntable's cartridge is old or damaged your records won't sound their best, and a worn stylus will degrade the sound of your precious LPs. If a stylus (needle) is readily available, replace the stylus for the cartridge that's mounted in the tonearm. Check the cartridge manufacturer's website for availability.
Better yet, buy a new phono cartridge, like the inexpensive Shure SC35C DJ cartridge that just received a rave review in Stereophile magazine. This cartridge is also rugged, so the stylus can survive a fair amount of abuse and still sound great, this one is highly recommended.
As for everybody else who listens to digital audio from a computer/laptop, I recommend the Hifiman HM-101 USB powered digital converter. You can listen to headphones through this little thing, or hook it up to a pair of desktop powered speakers, like Emotiva or M-Audio. The HM-101 is great for the money, but the Schiit Modi 2 digital converter is a big step up in clarity.
I've saved the best tip for last, it's the one you stick in your ears to protect your hearing: Etymotic Research Ety Plugs sell for a few bucks, but if they save your hearing at loud concerts and sports events, they're priceless. They are the best affordable earplugs I've tried, and I've tried a lot of plugs.
If you have suggestions for better sound on the cheap, share your ideas in the Comments section.