Longspur rattles are similar to each other. I find it useful to think of the notes of the Lapland Longspur rattle as being similar to the alarm call of the eastern Winter Wren (June, AK).
The Snow Bunting rattle is mellower and lower. Flock taking flight, also with buzzy calls (Jan, NJ).
rattle
buzzy flock call
probable alarm call at beginning of cut
The Smith's Longspur rattle is higher (June, AK).
McCown's Longspur has a squeaky rattle.
Other calls
Two Lapland Longspur calls (by males) I recorded on the breeding grounds in Alaska were a three-parted jer-hu and a buzzy call, also three-parted, described as foraging calls in BNA (June)
A less commonly heard call by Smith's Longspurs is an Eastern Meadowlark-like buzz (there is also a clear syu). (Jun, AK)
The common Chestnut-collared Longspur flight call is a cher-dee (bird in flight, May, ND).
Birds of the World Online previously The Birds of North America Online (BNA). Comprehensive information on all aspects of life history, including audio and video. By subscription.
Birdsong by the Seasons. Donald Kroodsma, Houghton Mifflin, 2009. Another great book with CDs from Dr Kroodsma.
Flight Calls of Migratory Birds WR Evans and M O'Brien. CD-ROM www.oldbird.org. Excellent source of information on flight calls
Nature's Music, The Science of Birdsong. Peter Marler and Hans Slabbekoorn. Elsevier Academic press. 2004. A treasure trove of information, advanced but very readable (comes with CD).
Peterson Field Guide to Bird Sounds of Eastern North America. Nathan Pieplow, 2017. Highly recommended, includes almost all known vocalizations of eastern US birds, with spectrograms and detailed analysis. Excellent!
Stokes Field Guide to Bird Song, Eastern Region, by Lang Elliot. 3 CDs. Of commercial recordings, this has the most comprehensive collection of songs and calls.
The Singing Life of Birds. Donald Kroodsma, Houghton Mifflin, 2005. Terrific book (plus CD), a must read.
The Songs of Wildbirds. Lang Elliott., Houghton Mifflin, 2005. Excellent photos with high quality recordings including some rare ones such as the flight song of the Western Meadowlark.
The Sound Approach to Birding. A guide to understanding bird sound. A. van de Berg, M Constantine, M. Robb. Superb discussion of bird calls and song with 2 CDs.