Since the speed of sound increases as the temperature increases, the actual speed of an object traveling at Mach 1 will depend on its altitude and the atmospheric conditions. The subject was first addressed in a question we answered on how fast is Mach 2 in miles per hour. The conclusion of that explanation was that the speed of sound is not a constant value. Instead, it changes depending on how high up in the atmosphere you are and on the temperature. To account for this behavior, aerospace engineers make use of what is called the standard atmosphere. This standard atmosphere is based on scientific atmospheric data collected at different locations within the atmosphere. This data was then used to create a series of equations that mathematically model the values of key atmospheric properties, such as temperature, density, and speed of sound. The results of this model provide engineers with "average" atmospheric properties on a so-called standard day. The standard atmospheric model tells us that the speed of sound, or Mach 1, at sea level is: 1,116.4 ft/s 340.29 m/s 761.18 mph 1,224.74 km/h 661.32 knots However, this model assumes a "standard day" in which the air temperature is about 60°F (15°C). If the actual temperature is higher, then the speed of sound will be higher as well. But the difference is small enough that we can neglect it for most engineering purposes, and the above values are accepted around the world as the speed of sound at sea level. But now we face another problem, because aircraft do not typically spend much time flying at sea level. They instead cruise tens of thousands of feet above the Earth's surface where the speed of sound changes. (NOTE: there is a picture/chart-can't paste it, so here is the link http://www.aerospaceweb.org/question/atmosphere/q0102c.shtml) Having established that temperature changes with altitude and speed of sound is directly proportional to temperature, it is now clear that the speed of sound changes as altitude increases. As illustrated above, the temperature decreases at a linear rate up to about 11 km (6.8 mi) where the Tropopause begins. This region of the atmosphere is marked by constant temperature, and therefore constant speed of sound. The Tropopause extends up to about 20 km (12.4 mi), so the speed of sound does not change throughout this entire 9 km (5.6 mi) thick region. Scientific measurements and the standard atmospheric model have established that the speed of sound, or Mach 1, within this realm is: 968.08 ft/s 295.07 m/s 660.05 mph 1,062.03 km/h 573.46 knots

Mach 1 is, by definition, the speed of sound. This varies with the atmospheric condition, depending on the medium characteristics and the temperature. Try this page for the equation and details: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html#c1

As stated before, Mach 1 varies, depending mainly on air density. Obviously, temperature and pressure are direct factors that affect density, so you could correctly say that it is dependant upon these. [Edit] Therefore: when taking measurements like this, scientists use a set of conditions called Standard Ambient Temperature and Pressure (STP). The official SATP is: 25 degrees C (77F) and 101.3kPa (29.92mmHg). [The other poster is correct. STP is 0 C and 1 atmosphere.] At STP, Mach 1 is 661knots (340m/s, 1224km/h, 765mph). The speed of sound decreases steadily as altitude increases, until it hits a point in the atmosphere called the tropopause. The tropopause is around 36,090 feet altitude. Above this height, temperature and pressure are fairly constant, so the speed of sound does not change. The temperature at 36,090ft is -57 degrees C (-70.6F). The speed of sound at and above the tropopause is 573kts (294.5m/s, 1060km/h, 662.5mph)

STP defined by the IUPAC is not 25C and 101.3kPa. It is 0C and 100kPa. There is also a quanity known as Standard Ambient Temperature & Pressure (SATP) also called Standard Laboratory Conditions (SLC), which is 25C and 100kPa. There was an older standard for STP which was 0C and 101.325kPa, and most chemical references from more than a decade ago refer to the earlier slightly higher pressure. The International Civil Aviation Organization defines a standard aviation reference of 101.325kPa at 15C and 0% humidity.

In order to get into a mach you would have to go 680 miles per hour.