I found a free version of a sound editing program called WavePad. There are two versions of the program; WavePad and WavePad Master's Edition. Master's is the one that costs money, but they "make WavePad free in the hope you will like it so much you will be tempted to upgrade to WavePad Master's Edition." As it states on their website, Master's Edition offers even more features than you already get from the free version.
The features include basic options like cutting, deleting, and copying pieces of an audio clip. You can also use the program to change the pitch and speed of either the whole audio clip or just little sections of it. You can also use some of the more advanced options that they offer in the free version. One of the ones that I thought was the most important was the noise reduction option, which is used to reduce background noise like skips and static sounds.
Some of the effects that are offered include amplify, normalize, equaliser, envelope, reverb, echo, reverse, etc. This can be very entertaining to play around with and can help add sound effects to your own voice clips. WavePad can also be used to convert cassette tapes or vinyl to a digital format. Most people probably don't even have a cassette let alone a vinyl record anymore, but if you do you could convert them to a more up-to-date format. This program mainly focuses around wav and mp3 files, but it can work with other formats like "vox, gsm, real audio, au, aif, flac, ogg and many more," as it lists on the website features section.