Music, music, music. It sets the tone and the mood for every event, no matter how big or small. Consider your guests' taste, the atmosphere you want create for your event, and make sure the music is at the right volume level (quiet enough for people to hear each other at a dinner party, loud enough for people to dance to on a dance floor).
Weddings + Music
Let's start at the ceremony space. While guests are busy arriving and getting seated (usually a good 30 minute period), consider playing songs/music during this time. It will set a nice ambiance, relax the mood, and it will get people excited for what's to come. If you don't have music playing, the only thing people will hear is the buzz of conversation, which could be slightly boring... and if you're the bride getting your final touches of lip gloss and hairspray in the back/side of the church or near the ceremony site, your nerves are probably a hot mess, so hearing some calming or sweet music will probably help to keep you happy and calm.
For the procession and recessional, it totally depends on your personal taste. You can pick any songs your heart desires! Classical music, songs that have meaningful lyrics, instrumental versions of any song, live performances/singers, etc. But if the ceremony is at a church, they may have strict rules about using only sacred/religious music, so keep that in mind and make sure you know the rules before the big day--you don't want any surprises thrown your way at the rehearsal!
As far as the reception goes, the biggest dilemma is DJ vs. live band. It's totally a personal preference, but no matter who you book, make sure you hear their sound and get referrals before you commit. We've been hearing (thanks to Pinterest and the "bridetip") about the option to send out cards with your invitations that give your guests the option to write in a song request for the reception. It's a very cute idea, but it also sounds like a DJ's worst nightmare! A few requests here and there are fine, but if you have 100+ guests all requesting songs, the DJ/band won't have any freedom to do their own thing. Just something to consider...
Dinner Parties + Music
If it's someone else's (or yours!) birthday/anniversary/special day: play the music he/she/they love! Ask the guest of honor beforehand what their favorite songs, bands, albums and genres are, and play that for the celebration! Just make sure to keep the vibe of the overall event in mind... if it's a small dinner party, keep the music low so people can actually converse. If it's a huge gathering, turn up the tunes a bit.
If the dinner party is just for fun, get some of all the guests' favorite music in there. You can mix it up with a lot of different genres to keep them guessing, or keep it all in the same family (Dean Martin, Frank Sinatra, etc. for a wine + cheese party)... just have fun with it!
Music makes every party better. Don't you think?!