Getting good sound is seldom a matter of equipment but distance and acoustic characteristics of the space. Any $2 mic, close to the sound source will beat the pants off a $2000 shotgun mic 30 feet away. So there are lots of options, for example a small digital recorder with built-in mic hidden in the flower arrangement 18 inches from the speaker in a seminar will do much better than the camera mounted or camera located mic. Another reason for sound pickup close, besides isolating the sound source from the highly reverberant acoustic space closer to the optical vantage point, is the difference in speed of sound versus light. The sound travels at a nominal 1100ft/sec so being 30 feet away means sound pick up will be a little less than 30milliseconds delayed from the visuals. That does not sound like much but we sense it and our brain tells us somethings is wrong. The delay becomes irritating even if we can't put our finger on what is causing our uneasiness. Get the mic close and that problem goes away, as does the terrible echo and reflection signals that dominate the waves reaching the mic when more than a couple feet from the source. Our brains filter much of that clutter out of our consciousness in a live acoustic space but the mic can't do that. The result is that familiar tinny, man in a cave sound that most people assume is due to a bad mic or recorder. Actually any mic will pick up that because it really is there even if we are not very aware of it when listening in the same room.
So, use a mic close the subject and be careful with levels and the quality and cost of the mic becomes less important. Getting too far away, 15 feet for example inside a building, and even the best mics will give poor results. The camera will accept 3.5mm phone plugs common on small electret-condenser mics so plug one in and place it close, making sure the cord is not in the frame. For more serious video recording, a sound engineer is needed and a boom operator to keep the mic close but out of frame. You really do not need fancy gear for good sound. The same electret-condenser mic element in a cheap computer mic(<$5) is used in more expensive video mics like the Rhode. The element itself costs $0.20 from Panasonic that is used in hundreds of models of mics from $5 to $200. Real quality mics are too fragile for most field recording, they are used in dialog replacement in a studio in post production. Due to the visual cuing, we are less critical about speech quality when it is appropriate for the scene and in sync, it will seem realistic even if the captured quality would be marginal when used in a audio only experience where we have to rely on ears alone to judge the content. Stan St Petersburg Russia