I use music and sounds during work for 2 reasons:

1. avoid distractions from noise in the office. That works best with a noise cancellation headset and if sounds are used which drown out the noise (e.g. the forest sound below is pretty effective for human voice)
2. help to get into the flow

So here are some sources which I use:

Sounds

These sites allow to create and share sound environments which are continously looped and randomized. You can also download the mixed samples.

• ambient-mixer.com
  Some of the sounds I mixed are energizing-ambience-2 Remix/Play&Download, harvest-jazz-venue---party-mix Remix/Play&Download, beach--seagulls-and-wind
• naturesoundsfor.me
  I created forest_anhu

Music

• lofi.cafe Multiple chillout / ambience music stations to pick
• musicforprogramming.net
• thechilloutloungemix.com
• EVE Online Music
  Music from a space MMOG if you like that stuff ... not everything on this page is good for the concentration, pick what works for you
• Moby Long Ambients